use strict;
use warnings;

sub subseqs {
   my @subseqs = ('');
   while ($_[0] =~ /(.)/sg) {
      push @subseqs, map "$_$1", @subseqs;
   }
   return \@subseqs;
}

sub common {
   my ($list1, $list2) = @_;
   my %common;
   for my $i1 (0..$#$list1) {
   for my $i2 (0..$#$list2) {
      if ($list1->[$i1] eq $list2->[$i2]) {
         $common{$list1->[$i1]} = 1;
      }
   }}
   return keys %common;
}

sub longest {
   my $longest = shift(@_);
   for (@_) {
      if (length($_) > length($longest)) {
         $longest = $_;
      }
   }
   return $longest;
}

{
   my $s1 = 'abdef';
   my $s2 = 'abcef';
   my $longest = longest(common(subseqs($s1), subseqs($s2)));
   print("$longest\n");
}