There is just one return list.

In the spirit of horrible solutions, you could pass the second one out-of-band. Right. But there's no sane reason to do that, is there?

So, in the spirit of insane solutions, have one more:

use strict;
use warnings;

sub getlists(\@\@);

getlists( my @a, my @b );
print join(" ",@a),"\n",join(" ",@b);

sub getlists(\@\@) {
    my ($foo, $bar) = @_;
    @$foo = (1,2,3);
    @$bar = (4,5,6);
}
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Most horrible yet? ;-)