He ran it and replies:
tony-******-macbook-pro:~ tony******$ perl --version
This is perl, v5.8.8 built for darwin-thread-multi-2level
(with 4 registered patches, see perl -V for more detail)
tony-******-macbook-pro:~ tony******$ perl m.perl
Name "main::FILE" used only once: possible typo at m.perl line 27.
Drag and drop a file here:
Invalid type 'W' in unpack at m.perl line 12, <STDIN> line 1.
Now, regarding your concern about extra bytes... plausible:
Note the space after x.txt... the square is probably my cursor.
Notes: (1) I anonymized his post by replacing his last name with asterisks, and (2) He included a screenshot that shows a blank after x.txt, which he alludes to in the last line. So his results agrees with the testing that you and louis.roca performed.