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(model4run1 model2run1 model4run2 model1run1);
print common(\%trie);