print map {"$_\n"} permute("abcd");

sub permute($) {
    my $s = shift;
    my @a;

    return ($s) unless (length($s)>1);

    for (my $i=0;$i<length($s);$i++) {
	my $r = $s;
	my $c = substr $r, $i, 1, "";
	push @a, map {"$c$_"} permute($r);
    }
    return @a;
}