Did you read the warning message? It talks about line 24. Which, in your code, is the following fragment:

@sorted_large_files =
    sort { -s "$_/$b" <=> -s "$_/$a" }
@large_files;
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Now, looking at this, there are three variables explicitly mentioned, $a, $b and $_. Looking at the documentation of sort, I find that $a and $b are set and used by the sort() function. Which leaves $_. This could be undefined. Maybe it would be a good idea to investigate the value of $_ and compare that with your expectations.

It's no real surprise that this works if there is only a single file, because then, the comparison block will never be called, BTW.