you should use substr instead of vec to extract characters from a string:

sub common {
  my ($a, $b)=@_;

  return ($a, '', '') if $a eq $b;

  for (my $i=0;;$i++) {
    my $c=substr($a, $i, 1);
    my $d=substr($b, $i, 1);
    return (substr($a, 0, $i), $c, $d)
       if $c ne $d;
  }
}
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