perlfaq7



PERLFAQ7(1)            Perl Programmers Reference Guide            PERLFAQ7(1)




NAME

       perlfaq7 - General Perl Language Issues ($Revision: 1.18 $, $Date:
       2004/11/03 22:54:08 $)


DESCRIPTION

       This section deals with general Perl language issues that don’t clearly
       fit into any of the other sections.

       Can I get a BNF/yacc/RE for the Perl language?

       There is no BNF, but you can paw your way through the yacc grammar in
       perly.y in the source distribution if you’re particularly brave.  The
       grammar relies on very smart tokenizing code, so be prepared to venture
       into toke.c as well.

       In the words of Chaim Frenkel: "Perl’s grammar can not be reduced to
       BNF.  The work of parsing perl is distributed between yacc, the lexer,
       smoke and mirrors."

       What are all these $@%&* punctuation signs, and how do I know when to
       use them?

       They are type specifiers, as detailed in perldata:

           $ for scalar values (number, string or reference)
           @ for arrays
           % for hashes (associative arrays)
           & for subroutines (aka functions, procedures, methods)
           * for all types of that symbol name.  In version 4 you used them like
             pointers, but in modern perls you can just use references.

       There are couple of other symbols that you’re likely to encounter that
       aren’t really type specifiers:

           <> are used for inputting a record from a filehandle.
           \  takes a reference to something.

       Note that <FILE> is neither the type specifier for files nor the name
       of the handle.  It is the "<>" operator applied to the handle FILE.  It
       reads one line (well, record--see "$/" in perlvar) from the handle FILE
       in scalar context, or all lines in list context.  When performing open,
       close, or any other operation besides "<>" on files, or even when talk-
       ing about the handle, do not use the brackets.  These are correct:
       "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN to FILE".

       Do I always/never have to quote my strings or use semicolons and com-
       mas?

       Normally, a bareword doesn’t need to be quoted, but in most cases prob-
       ably should be (and must be under "use strict").  But a hash key con-
       sisting of a simple word (that isn’t the name of a defined subroutine)
       and the left-hand operand to the "=>" operator both count as though
       they were quoted:

           This                    is like this
           ------------            ---------------
           $foo{line}              $foo{"line"}
           bar => stuff            "bar" => stuff

       The final semicolon in a block is optional, as is the final comma in a
       list.  Good style (see perlstyle) says to put them in except for
       one-liners:

           if ($whoops) { exit 1 }
           @nums = (1, 2, 3);

           if ($whoops) {
               exit 1;
           }
           @lines = (
               "There Beren came from mountains cold",
               "And lost he wandered under leaves",
           );

       How do I skip some return values?

       One way is to treat the return values as a list and index into it:

               $dir = (getpwnam($user))[7];

       Another way is to use undef as an element on the left-hand-side:

           ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

       You can also use a list slice to select only the elements that you
       need:

               ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];

       How do I temporarily block warnings?

       If you are running Perl 5.6.0 or better, the "use warnings" pragma
       allows fine control of what warning are produced.  See perllexwarn for
       more details.

           {
               no warnings;          # temporarily turn off warnings
               $a = $b + $c;         # I know these might be undef
           }

       Additionally, you can enable and disable categories of warnings.  You
       turn off the categories you want to ignore and you can still get other
       categories of warnings.  See perllexwarn for the complete details,
       including the category names and hierarchy.

               {
               no warnings ’uninitialized’;
               $a = $b + $c;
               }

       If you have an older version of Perl, the $^W variable (documented in
       perlvar) controls runtime warnings for a block:

           {
               local $^W = 0;        # temporarily turn off warnings
               $a = $b + $c;         # I know these might be undef
           }

       Note that like all the punctuation variables, you cannot currently use
       my() on $^W, only local().

       Whats an extension?

       An extension is a way of calling compiled C code from Perl.  Reading
       perlxstut is a good place to learn more about extensions.

       Why do Perl operators have different precedence than C operators?

       Actually, they don’t.  All C operators that Perl copies have the same
       precedence in Perl as they do in C.  The problem is with operators that
       C doesn’t have, especially functions that give a list context to every-
       thing on their right, eg. print, chmod, exec, and so on.  Such func-
       tions are called "list operators" and appear as such in the precedence
       table in perlop.

       A common mistake is to write:

           unlink $file ││ die "snafu";

       This gets interpreted as:

           unlink ($file ││ die "snafu");

       To avoid this problem, either put in extra parentheses or use the super
       low precedence "or" operator:

           (unlink $file) ││ die "snafu";
           unlink $file or die "snafu";

       The "English" operators ("and", "or", "xor", and "not") deliberately
       have precedence lower than that of list operators for just such situa-
       tions as the one above.

       Another operator with surprising precedence is exponentiation.  It
       binds more tightly even than unary minus, making "-2**2" product a neg-
       ative not a positive four.  It is also right-associating, meaning that
       "2**3**2" is two raised to the ninth power, not eight squared.

       Although it has the same precedence as in C, Perl’s "?:" operator pro-
       duces an lvalue.  This assigns $x to either $a or $b, depending on the
       trueness of $maybe:

           ($maybe ? $a : $b) = $x;

       How do I declare/create a structure?

       In general, you don’t "declare" a structure.  Just use a (probably
       anonymous) hash reference.  See perlref and perldsc for details.
       Here’s an example:

           $person = {};                   # new anonymous hash
           $person->{AGE}  = 24;           # set field AGE to 24
           $person->{NAME} = "Nat";        # set field NAME to "Nat"

       If you’re looking for something a bit more rigorous, try perltoot.

       How do I create a module?

       A module is a package that lives in a file of the same name.  For exam-
       ple, the Hello::There module would live in Hello/There.pm.  For
       details, read perlmod.  You’ll also find Exporter helpful.  If you’re
       writing a C or mixed-language module with both C and Perl, then you
       should study perlxstut.

       The "h2xs" program will create stubs for all the important stuff for
       you:

         % h2xs -XA -n My::Module

       The "-X" switch tells "h2xs" that you are not using "XS" extension
       code.  The "-A" switch tells "h2xs" that you are not using the
       AutoLoader, and the "-n" switch specifies the name of the module.  See
       h2xs for more details.

       How do I create a class?

       See perltoot for an introduction to classes and objects, as well as
       perlobj and perlbot.

       How can I tell if a variable is tainted?

       You can use the tainted() function of the Scalar::Util module, avail-
       able from CPAN (or included with Perl since release 5.8.0).  See also
       "Laundering and Detecting Tainted Data" in perlsec.

       Whats a closure?

       Closures are documented in perlref.

       Closure is a computer science term with a precise but hard-to-explain
       meaning. Closures are implemented in Perl as anonymous subroutines with
       lasting references to lexical variables outside their own scopes.
       These lexicals magically refer to the variables that were around when
       the subroutine was defined (deep binding).

       Closures make sense in any programming language where you can have the
       return value of a function be itself a function, as you can in Perl.
       Note that some languages provide anonymous functions but are not capa-
       ble of providing proper closures: the Python language, for example.
       For more information on closures, check out any textbook on functional
       programming.  Scheme is a language that not only supports but encour-
       ages closures.

       Here’s a classic function-generating function:

           sub add_function_generator {
             return sub { shift() + shift() };
           }

           $add_sub = add_function_generator();
           $sum = $add_sub->(4,5);                # $sum is 9 now.

       The closure works as a function template with some customization slots
       left out to be filled later.  The anonymous subroutine returned by
       add_function_generator() isn’t technically a closure because it refers
       to no lexicals outside its own scope.

       Contrast this with the following make_adder() function, in which the
       returned anonymous function contains a reference to a lexical variable
       outside the scope of that function itself.  Such a reference requires
       that Perl return a proper closure, thus locking in for all time the
       value that the lexical had when the function was created.

           sub make_adder {
               my $addpiece = shift;
               return sub { shift() + $addpiece };
           }

           $f1 = make_adder(20);
           $f2 = make_adder(555);

       Now "&$f1($n)" is always 20 plus whatever $n you pass in, whereas
       "&$f2($n)" is always 555 plus whatever $n you pass in.  The $addpiece
       in the closure sticks around.

       Closures are often used for less esoteric purposes.  For example, when
       you want to pass in a bit of code into a function:

           my $line;
           timeout( 30, sub { $line = <STDIN> } );

       If the code to execute had been passed in as a string, ’$line =
       <STDIN>’, there would have been no way for the hypothetical timeout()
       function to access the lexical variable $line back in its caller’s
       scope.

       What is variable suicide and how can I prevent it?

       Variable suicide is when you (temporarily or permanently) lose the
       value of a variable.  It is caused by scoping through my() and local()
       interacting with either closures or aliased foreach() iterator vari-
       ables and subroutine arguments.  It used to be easy to inadvertently
       lose a variable’s value this way, but now it’s much harder.  Take this
       code:

           my $f = "foo";
           sub T {
             while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
           }
           T;
           print "Finally $f\n";

       The $f that has "bar" added to it three times should be a new $f ("my
       $f" should create a new local variable each time through the loop).  It
       isn’t, however.  This was a bug, now fixed in the latest releases
       (tested against 5.004_05, 5.005_03, and 5.005_56).

       How can I pass/return a {Function, FileHandle, Array, Hash, Method,
       Regex}?

       With the exception of regexes, you need to pass references to these
       objects.  See "Pass by Reference" in perlsub for this particular ques-
       tion, and perlref for information on references.

       See ‘‘Passing Regexes’’, below, for information on passing regular
       expressions.

       Passing Variables and Functions
           Regular variables and functions are quite easy to pass: just pass
           in a reference to an existing or anonymous variable or function:

               func( \$some_scalar );

               func( \@some_array  );
               func( [ 1 .. 10 ]   );

               func( \%some_hash   );
               func( { this => 10, that => 20 }   );

               func( \&some_func   );
               func( sub { $_[0] ** $_[1] }   );

       Passing Filehandles
           As of Perl 5.6, you can represent filehandles with scalar variables
           which you treat as any other scalar.

                   open my $fh, $filename or die "Cannot open $filename! $!";
                   func( $fh );

                   sub func {
                           my $passed_fh = shift;

                           my $line = <$fh>;
                           }

           Before Perl 5.6, you had to use the *FH or "\*FH" notations.  These
           are "typeglobs"--see "Typeglobs and Filehandles" in perldata and
           especially "Pass by Reference" in perlsub for more information.

       Passing Regexes
           To pass regexes around, you’ll need to be using a release of Perl
           sufficiently recent as to support the "qr//" construct, pass around
           strings and use an exception-trapping eval, or else be very, very
           clever.

           Here’s an example of how to pass in a string to be regex compared
           using "qr//":

               sub compare($$) {
                   my ($val1, $regex) = @_;
                   my $retval = $val1 =~ /$regex/;
                   return $retval;
               }
               $match = compare("old McDonald", qr/d.*D/i);

           Notice how "qr//" allows flags at the end.  That pattern was com-
           piled at compile time, although it was executed later.  The nifty
           "qr//" notation wasn’t introduced until the 5.005 release.  Before
           that, you had to approach this problem much less intuitively.  For
           example, here it is again if you don’t have "qr//":

               sub compare($$) {
                   my ($val1, $regex) = @_;
                   my $retval = eval { $val1 =~ /$regex/ };
                   die if $@;
                   return $retval;
               }

               $match = compare("old McDonald", q/($?i)d.*D/);

           Make sure you never say something like this:

               return eval "\$val =~ /$regex/";   # WRONG

           or someone can sneak shell escapes into the regex due to the double
           interpolation of the eval and the double-quoted string.  For exam-
           ple:

               $pattern_of_evil = ’danger ${ system("rm -rf * &") } danger’;

               eval "\$string =~ /$pattern_of_evil/";

           Those preferring to be very, very clever might see the O’Reilly
           book, Mastering Regular Expressions, by Jeffrey Friedl.  Page 273’s
           Build_MatchMany_Function() is particularly interesting.  A complete
           citation of this book is given in perlfaq2.

       Passing Methods
           To pass an object method into a subroutine, you can do this:

               call_a_lot(10, $some_obj, "methname")
               sub call_a_lot {
                   my ($count, $widget, $trick) = @_;
                   for (my $i = 0; $i < $count; $i++) {
                       $widget->$trick();
                   }
               }

           Or, you can use a closure to bundle up the object, its method call,
           and arguments:

               my $whatnot =  sub { $some_obj->obfuscate(@args) };
               func($whatnot);
               sub func {
                   my $code = shift;
                   &$code();
               }

           You could also investigate the can() method in the UNIVERSAL class
           (part of the standard perl distribution).

       How do I create a static variable?

       As with most things in Perl, TMTOWTDI.  What is a "static variable" in
       other languages could be either a function-private variable (visible
       only within a single function, retaining its value between calls to
       that function), or a file-private variable (visible only to functions
       within the file it was declared in) in Perl.

       Here’s code to implement a function-private variable:

           BEGIN {
               my $counter = 42;
               sub prev_counter { return --$counter }
               sub next_counter { return $counter++ }
           }

       Now prev_counter() and next_counter() share a private variable $counter
       that was initialized at compile time.

       To declare a file-private variable, you’ll still use a my(), putting
       the declaration at the outer scope level at the top of the file.
       Assume this is in file Pax.pm:

           package Pax;
           my $started = scalar(localtime(time()));

           sub begun { return $started }

       When "use Pax" or "require Pax" loads this module, the variable will be
       initialized.  It won’t get garbage-collected the way most variables
       going out of scope do, because the begun() function cares about it, but
       no one else can get it.  It is not called $Pax::started because its
       scope is unrelated to the package.  It’s scoped to the file.  You could
       conceivably have several packages in that same file all accessing the
       same private variable, but another file with the same package couldn’t
       get to it.

       See "Persistent Private Variables" in perlsub for details.

       Whats the difference between dynamic and lexical (static) scoping?
       Between local() and my()?

       "local($x)" saves away the old value of the global variable $x and
       assigns a new value for the duration of the subroutine which is visible
       in other functions called from that subroutine.  This is done at
       run-time, so is called dynamic scoping.  local() always affects global
       variables, also called package variables or dynamic variables.

       "my($x)" creates a new variable that is only visible in the current
       subroutine.  This is done at compile-time, so it is called lexical or
       static scoping.  my() always affects private variables, also called
       lexical variables or (improperly) static(ly scoped) variables.

       For instance:

           sub visible {
               print "var has value $var\n";
           }

           sub dynamic {
               local $var = ’local’;   # new temporary value for the still-global
               visible();              #   variable called $var
           }

           sub lexical {
               my $var = ’private’;    # new private variable, $var
               visible();              # (invisible outside of sub scope)
           }

           $var = ’global’;

           visible();                  # prints global
           dynamic();                  # prints local
           lexical();                  # prints global

       Notice how at no point does the value "private" get printed.  That’s
       because $var only has that value within the block of the lexical()
       function, and it is hidden from called subroutine.

       In summary, local() doesn’t make what you think of as private, local
       variables.  It gives a global variable a temporary value.  my() is what
       you’re looking for if you want private variables.

       See "Private Variables via my()" in perlsub and "Temporary Values via
       local()" in perlsub for excruciating details.

       How can I access a dynamic variable while a similarly named lexical is
       in scope?

       If you know your package, you can just mention it explicitly, as in
       $Some_Pack::var. Note that the notation $::var is not the dynamic $var
       in the current package, but rather the one in the "main" package, as
       though you had written $main::var.

               use vars ’$var’;
               local $var = "global";
               my    $var = "lexical";

               print "lexical is $var\n";
               print "global  is $main::var\n";

       Alternatively you can use the compiler directive our() to bring a
       dynamic variable into the current lexical scope.

               require 5.006; # our() did not exist before 5.6
               use vars ’$var’;

               local $var = "global";
               my $var    = "lexical";

               print "lexical is $var\n";

               {
                 our $var;
                 print "global  is $var\n";
               }

       Whats the difference between deep and shallow binding?

       In deep binding, lexical variables mentioned in anonymous subroutines
       are the same ones that were in scope when the subroutine was created.
       In shallow binding, they are whichever variables with the same names
       happen to be in scope when the subroutine is called.  Perl always uses
       deep binding of lexical variables (i.e., those created with my()).
       However, dynamic variables (aka global, local, or package variables)
       are effectively shallowly bound.  Consider this just one more reason
       not to use them.  See the answer to "What’s a closure?".

       Why doesnt "my($foo) = <FILE>;" work right?

       "my()" and "local()" give list context to the right hand side of "=".
       The <FH> read operation, like so many of Perl’s functions and opera-
       tors, can tell which context it was called in and behaves appropri-
       ately.  In general, the scalar() function can help.  This function does
       nothing to the data itself (contrary to popular myth) but rather tells
       its argument to behave in whatever its scalar fashion is.  If that
       function doesn’t have a defined scalar behavior, this of course doesn’t
       help you (such as with sort()).

       To enforce scalar context in this particular case, however, you need
       merely omit the parentheses:

           local($foo) = <FILE>;           # WRONG
           local($foo) = scalar(<FILE>);   # ok
           local $foo  = <FILE>;           # right

       You should probably be using lexical variables anyway, although the
       issue is the same here:

           my($foo) = <FILE>;  # WRONG
           my $foo  = <FILE>;  # right

       How do I redefine a builtin function, operator, or method?

       Why do you want to do that? :-)

       If you want to override a predefined function, such as open(), then
       you’ll have to import the new definition from a different module.  See
       "Overriding Built-in Functions" in perlsub.  There’s also an example in
       "Class::Template" in perltoot.

       If you want to overload a Perl operator, such as "+" or "**", then
       you’ll want to use the "use overload" pragma, documented in overload.

       If you’re talking about obscuring method calls in parent classes, see
       "Overridden Methods" in perltoot.

       Whats the difference between calling a function as &foo and foo()?

       When you call a function as &foo, you allow that function access to
       your current @_ values, and you bypass prototypes.  The function
       doesn’t get an empty @_--it gets yours!  While not strictly speaking a
       bug (it’s documented that way in perlsub), it would be hard to consider
       this a feature in most cases.

       When you call your function as "&foo()", then you do get a new @_, but
       prototyping is still circumvented.

       Normally, you want to call a function using "foo()".  You may only omit
       the parentheses if the function is already known to the compiler
       because it already saw the definition ("use" but not "require"), or via
       a forward reference or "use subs" declaration.  Even in this case, you
       get a clean @_ without any of the old values leaking through where they
       don’t belong.

       How do I create a switch or case statement?

       This is explained in more depth in the perlsyn.  Briefly, there’s no
       official case statement, because of the variety of tests possible in
       Perl (numeric comparison, string comparison, glob comparison, regex
       matching, overloaded comparisons, ...).  Larry couldn’t decide how best
       to do this, so he left it out, even though it’s been on the wish list
       since perl1.

       Starting from Perl 5.8 to get switch and case one can use the Switch
       extension and say:

               use Switch;

       after which one has switch and case.  It is not as fast as it could be
       because it’s not really part of the language (it’s done using source
       filters) but it is available, and it’s very flexible.

       But if one wants to use pure Perl, the general answer is to write a
       construct like this:

           for ($variable_to_test) {
               if    (/pat1/)  { }     # do something
               elsif (/pat2/)  { }     # do something else
               elsif (/pat3/)  { }     # do something else
               else            { }     # default
           }

       Here’s a simple example of a switch based on pattern matching, this
       time lined up in a way to make it look more like a switch statement.
       We’ll do a multiway conditional based on the type of reference stored
       in $whatchamacallit:

           SWITCH: for (ref $whatchamacallit) {

               /^$/            && die "not a reference";

               /SCALAR/        && do {
                                       print_scalar($$ref);
                                       last SWITCH;
                               };

               /ARRAY/         && do {
                                       print_array(@$ref);
                                       last SWITCH;
                               };

               /HASH/          && do {
                                       print_hash(%$ref);
                                       last SWITCH;
                               };

               /CODE/          && do {
                                       warn "can’t print function ref";
                                       last SWITCH;
                               };

               # DEFAULT

               warn "User defined type skipped";

           }

       See "perlsyn/"Basic BLOCKs and Switch Statements"" for many other exam-
       ples in this style.

       Sometimes you should change the positions of the constant and the vari-
       able.  For example, let’s say you wanted to test which of many answers
       you were given, but in a case-insensitive way that also allows abbrevi-
       ations.  You can use the following technique if the strings all start
       with different characters or if you want to arrange the matches so that
       one takes precedence over another, as "SEND" has precedence over "STOP"
       here:

           chomp($answer = <>);
           if    ("SEND"  =~ /^\Q$answer/i) { print "Action is send\n"  }
           elsif ("STOP"  =~ /^\Q$answer/i) { print "Action is stop\n"  }
           elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
           elsif ("LIST"  =~ /^\Q$answer/i) { print "Action is list\n"  }
           elsif ("EDIT"  =~ /^\Q$answer/i) { print "Action is edit\n"  }

       A totally different approach is to create a hash of function refer-
       ences.

           my %commands = (
               "happy" => \&joy,
               "sad",  => \&sullen,
               "done"  => sub { die "See ya!" },
               "mad"   => \&angry,
           );

           print "How are you? ";
           chomp($string = <STDIN>);
           if ($commands{$string}) {
               $commands{$string}->();
           } else {
               print "No such command: $string\n";
           }

       How can I catch accesses to undefined variables, functions, or methods?

       The AUTOLOAD method, discussed in "Autoloading" in perlsub and
       "AUTOLOAD: Proxy Methods" in perltoot, lets you capture calls to unde-
       fined functions and methods.

       When it comes to undefined variables that would trigger a warning under
       "use warnings", you can promote the warning to an error.

               use warnings FATAL => qw(uninitialized);

       Why cant a method included in this same file be found?

       Some possible reasons: your inheritance is getting confused, you’ve
       misspelled the method name, or the object is of the wrong type.  Check
       out perltoot for details about any of the above cases.  You may also
       use "print ref($object)" to find out the class $object was blessed
       into.

       Another possible reason for problems is because you’ve used the indi-
       rect object syntax (eg, "find Guru "Samy"") on a class name before Perl
       has seen that such a package exists.  It’s wisest to make sure your
       packages are all defined before you start using them, which will be
       taken care of if you use the "use" statement instead of "require".  If
       not, make sure to use arrow notation (eg., "Guru->find("Samy")")
       instead.  Object notation is explained in perlobj.

       Make sure to read about creating modules in perlmod and the perils of
       indirect objects in "Method Invocation" in perlobj.

       How can I find out my current package?

       If you’re just a random program, you can do this to find out what the
       currently compiled package is:

           my $packname = __PACKAGE__;

       But, if you’re a method and you want to print an error message that
       includes the kind of object you were called on (which is not necessar-
       ily the same as the one in which you were compiled):

           sub amethod {
               my $self  = shift;
               my $class = ref($self) ││ $self;
               warn "called me from a $class object";
           }

       How can I comment out a large block of perl code?

       You can use embedded POD to discard it.  Enclose the blocks you want to
       comment out in POD markers, for example "=for nobody" and "=cut" (which
       marks ends of POD blocks).

           # program is here

           =for nobody

           all of this stuff

           here will be ignored
           by everyone

           =cut

           # program continues

       The pod directives cannot go just anywhere.  You must put a pod direc-
       tive where the parser is expecting a new statement, not just in the
       middle of an expression or some other arbitrary grammar production.

       See perlpod for more details.

       How do I clear a package?

       Use this code, provided by Mark-Jason Dominus:

           sub scrub_package {
               no strict ’refs’;
               my $pack = shift;
               die "Shouldn’t delete main package"
                   if $pack eq "" ││ $pack eq "main";
               my $stash = *{$pack . ’::’}{HASH};
               my $name;
               foreach $name (keys %$stash) {
                   my $fullname = $pack . ’::’ . $name;
                   # Get rid of everything with that name.
                   undef $$fullname;
                   undef @$fullname;
                   undef %$fullname;
                   undef &$fullname;
                   undef *$fullname;
               }
           }

       Or, if you’re using a recent release of Perl, you can just use the Sym-
       bol::delete_package() function instead.

       How can I use a variable as a variable name?

       Beginners often think they want to have a variable contain the name of
       a variable.

           $fred    = 23;
           $varname = "fred";
           ++$$varname;         # $fred now 24

       This works sometimes, but it is a very bad idea for two reasons.

       The first reason is that this technique only works on global variables.
       That means that if $fred is a lexical variable created with my() in the
       above example, the code wouldn’t work at all: you’d accidentally access
       the global and skip right over the private lexical altogether.  Global
       variables are bad because they can easily collide accidentally and in
       general make for non-scalable and confusing code.

       Symbolic references are forbidden under the "use strict" pragma.  They
       are not true references and consequently are not reference counted or
       garbage collected.

       The other reason why using a variable to hold the name of another vari-
       able is a bad idea is that the question often stems from a lack of
       understanding of Perl data structures, particularly hashes.  By using
       symbolic references, you are just using the package’s symbol-table hash
       (like %main::) instead of a user-defined hash.  The solution is to use
       your own hash or a real reference instead.

           $USER_VARS{"fred"} = 23;
           $varname = "fred";
           $USER_VARS{$varname}++;  # not $$varname++

       There we’re using the %USER_VARS hash instead of symbolic references.
       Sometimes this comes up in reading strings from the user with variable
       references and wanting to expand them to the values of your perl pro-
       gram’s variables.  This is also a bad idea because it conflates the
       program-addressable namespace and the user-addressable one.  Instead of
       reading a string and expanding it to the actual contents of your pro-
       gram’s own variables:

           $str = ’this has a $fred and $barney in it’;
           $str =~ s/(\$\w+)/$1/eeg;             # need double eval

       it would be better to keep a hash around like %USER_VARS and have vari-
       able references actually refer to entries in that hash:

           $str =~ s/\$(\w+)/$USER_VARS{$1}/g;   # no /e here at all

       That’s faster, cleaner, and safer than the previous approach.  Of
       course, you don’t need to use a dollar sign.  You could use your own
       scheme to make it less confusing, like bracketed percent symbols, etc.

           $str = ’this has a %fred% and %barney% in it’;
           $str =~ s/%(\w+)%/$USER_VARS{$1}/g;   # no /e here at all

       Another reason that folks sometimes think they want a variable to con-
       tain the name of a variable is because they don’t know how to build
       proper data structures using hashes.  For example, let’s say they
       wanted two hashes in their program: %fred and %barney, and that they
       wanted to use another scalar variable to refer to those by name.

           $name = "fred";
           $$name{WIFE} = "wilma";     # set %fred

           $name = "barney";
           $$name{WIFE} = "betty";     # set %barney

       This is still a symbolic reference, and is still saddled with the prob-
       lems enumerated above.  It would be far better to write:

           $folks{"fred"}{WIFE}   = "wilma";
           $folks{"barney"}{WIFE} = "betty";

       And just use a multilevel hash to start with.

       The only times that you absolutely must use symbolic references are
       when you really must refer to the symbol table.  This may be because
       it’s something that can’t take a real reference to, such as a format
       name.  Doing so may also be important for method calls, since these
       always go through the symbol table for resolution.

       In those cases, you would turn off "strict ’refs’" temporarily so you
       can play around with the symbol table.  For example:

           @colors = qw(red blue green yellow orange purple violet);
           for my $name (@colors) {
               no strict ’refs’;  # renege for the block
               *$name = sub { "<FONT COLOR=’$name’>@_</FONT>" };
           }

       All those functions (red(), blue(), green(), etc.) appear to be sepa-
       rate, but the real code in the closure actually was compiled only once.

       So, sometimes you might want to use symbolic references to directly
       manipulate the symbol table.  This doesn’t matter for formats, handles,
       and subroutines, because they are always global--you can’t use my() on
       them.  For scalars, arrays, and hashes, though--and usually for subrou-
       tines-- you probably only want to use hard references.

       What does "bad interpreter" mean?

       The "bad interpreter" message comes from the shell, not perl.  The
       actual message may vary depending on your platform, shell, and locale
       settings.

       If you see "bad interpreter - no such file or directory", the first
       line in your perl script (the "shebang" line) does not contain the
       right path to perl (or any other program capable of running scripts).
       Sometimes this happens when you move the script from one machine to
       another and each machine has a different path to perl---/usr/bin/perl
       versus /usr/local/bin/perl for instance.

       If you see "bad interpreter: Permission denied", you need to make your
       script executable.

       In either case, you should still be able to run the scripts with perl
       explicitly:

               % perl script.pl

       If you get a message like "perl: command not found", perl is not in
       your PATH, which might also mean that the location of perl is not where
       you expect it so you need to adjust your shebang line.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.  All
       rights reserved.

       This documentation is free; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in this file are
       hereby placed into the public domain.  You are permitted and encouraged
       to use this code in your own programs for fun or for profit as you see
       fit.  A simple comment in the code giving credit would be courteous but
       is not required.



perl v5.8.6                       2004-11-05                       PERLFAQ7(1)

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