my $posn = -1;
my $same = 1;
while(defined $same){
  die if $posn > length $files[0];
  my $chr = substr($files[0], ++$posn, 1);
  for my $name (1..$#files) {
    undef $same and last if(substr($files[$name], $posn, 1) ne $chr 
                                  or length $files[$name] < $posn);
  }
}

print 'Prefix is "', substr($files[0], 0, $posn), '"';