elef:

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)
<<snip>>

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:
/Users/tony******/Desktop/x.txt 
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.
[download]

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.

...roboticus