The Perl grep returns the matching contents from the file, not the file name, so your code should be more like:

opendir (DIR, "$responseDir") or die "could not open $responseDir :$!\
+n";
foreach $file ( readdir(DIR) ) {
  open THISFILE, "$responseDir/$file";
  my @newFile = grep { /$commonField/ } <THISFILE>;
  if ( @newFile ) {
    # If there were any contents, this is our file.
    $matchedfile = $file;
    close THISFILE;
    last;
  }
  close THISFILE;
}
closedir(DIR);
print "Matched file $matchedfile.\n";
[download]

I tested a slight variation of this. You'll definitely want to re-test this, but I think it works.

--J