## This is what I think is needed beforehand :
open( TIDY, "html-tidy $commandline|") or die "Couldn't spawn html-tidy : $!\n";

my @output;
@output = <TIDY>;

## Here begins their code :
if (close(TIDY) == 0) {
  my $exitcode = $? >> 8;
  if ($exitcode == 1) {
    printf STDERR "tidy issued warning messages\n";
  } elsif ($exitcode == 2) {
    printf STDERR "tidy issued error messages\n";
  } else {
    die "tidy exited with code: $exitcode\n";
  }
} else {
  printf STDERR "tidy detected no errors\n";
}