gistfile1.pl

#!/usr/bin/perl

use strict;
use warnings;

sub aa {

    my $aa = 123 if not 1;

    if (defined $aa) {
        print $aa . "\n";
    } else {
        print "undef\n";
    }

    $aa = rand();
}

foreach (1..5) {
    aa();
}
__END__

The output is something like:

    undef
    0.6073304606407
    0.922599598623009
    0.141934602326852
    0.169187877417194