Ah, yes--that makes sense! Used the hours calculation for all four snippets:

$amount = $amount * 60 * 60;
[download]

and got identical results, until 79 hours was used (a result difference of 2), but I suspect the division in the grep { -M >= $amount / 86400 } yields a different precision.

Interestingly, IO::All didn't see the file "song自然之歌," but File::Find::Rule did (and should have).