c:\@Work\Perl\monks\Albannach>perl -wMstrict -le
"sub insert {
   my $trie=shift;
   my $str=shift;
   $trie=$trie->{$_}||={}
     foreach (split //,$str);
 }
 ;;
 sub common {
   my $trie=shift;
   my $common=\"\";
   while (1==scalar keys %$trie) {
       my $char=(keys %$trie)[0];
       $common.=$char;
       $trie=$trie->{$char};
   }
   $common;
 }
 ;;
 my %trie;
 insert(\%trie,$_) foreach qw(a ab abc);
 print common(\%trie);
"
abc