To do it your way, you need to read in the contents of the file and use the /s modifier on your regular expression:

use File::Slurp;
...
for my $file (@files) {
  my $content = read_file($file);

  unless ($content =~ m{<order>(.*)</order>}s) {
    # bad file
  } else {
    # found <order>...</order>
  }
}
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Without the /s modifier, a dot (.) will not match a newline.