Your $module_id (which is what I take it your mean by "a value contained within the while loop") is declared inside the loop so scope ends when the loop ends. To retain those values push them onto something declared outside the loop.

#!/usr/bin/env perl 
use strict;
use warnings;
use Data::Dumper;

my @retarr = ();

print "At init " . Dumper (\@retarr);
@retarr = void ();
print "After void " . Dumper (\@retarr);
@retarr = getlist ();
print "After getlist " . Dumper (\@retarr);

exit;

sub void {
    # doesn't return anything
}

sub getlist {
    my @foo;
    while ($#foo < 2) {
        my $module_id = rand ();
        push @foo, $module_id;
    }
    return @foo;
}
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