Perl Language Variables Typeglobs, typeglob refs, filehandles and constants


Example

A typeglob *foo holds references to the contents of global variables with that name: $foo, @foo, $foo, &foo, etc. You can access it like an hash and assign to manipulate the symbol tables directly (evil!).

use v5.10; # necessary for say
our $foo = "foo";
our $bar;
say ref *foo{SCALAR};     # SCALAR
say ${ *foo{SCALAR} };    # bar
*bar = *foo;
say $bar;                 # bar
$bar = 'egg';
say $foo;                 # egg

Typeglobs are more commonly handled when dealing with files. open, for example, produces a reference to a typeglob when asked to create a non-global filehandle:

use v5.10; # necessary for say
open(my $log, '> utf-8', '/tmp/log') or die $!; # open for writing with encoding
say $log 'Log opened';

# You can dereference this globref, but it's not very useful.
say ref $log;                   # GLOB
say (*{$log}->{IO} // 'undef'); # undef

close $log or die $!;

Typeglobs can also be used to make global read-only variables, though use constant is in broader use.

# Global constant creation
*TRUE = \('1');
our $TRUE;
say $TRUE;  # 1
$TRUE = ''; # dies, "Modification of a read-only value attempted"

# use constant instead defines a parameterless function, therefore it's not global,
# can be used without sigils, can be imported, but does not interpolate easily.
use constant (FALSE => 0);
say FALSE;        # 0
say &FALSE;       # 0
say "${\FALSE}";  # 0 (ugh)
say *FALSE{CODE}; # CODE(0xMA1DBABE)

# Of course, neither is truly constant when you can manipulate the symbol table...
*TRUE = \('');
use constant (EVIL => 1);
*FALSE = *EVIL;