Question

I encountered some strange behavior which hints that I do not understand some basic things about Perl script execution and initialization order. The following example:

#!/usr/bin/env perl

use strict;
use warnings;

print &get_keys(), "\n";

use vars qw/%hello/;  # same effect with 'my %hello = (...);'
%hello = ( a => 1, b => 2 );

sub get_keys
{
        return join(', ', sort keys %hello);
}

prints an empty string. Meaning that though variable is already visible, since the state with assignment wasn’t yet reached, it has no value. (Using a scalar instead of the hash would trigger a warning about uninitialized variable.)

Is that intended behavior?

I would be also glad for the RTFM pointers.

Answer 1

From perlsub:

A my has both a compile-time and a
run-time effect. At compile time, the
compiler takes notice of it. The
principal usefulness of this is to
quiet use strict 'vars' …. Actual
initialization is delayed until run
time, though, so it gets executed at
the appropriate time.

# At this point, %hello is a lexically scope variable (the my took effect
# at compile time), but it still has no keys.
print get_keys();

my %hello = ( a => 1, b => 2 );

# Now the hash has content.
print get_keys();

sub get_keys { join ' ', keys %hello, "\n" }

Other notes: (1) You should be using my or our rather than use vars. (2) Under normal circumstances, don’t call functions with a leading ampersand: use foo() rather than &foo().