Your map is returning a list (Update Hmmm, maybe not so much). If you want a reference to an array containing that list, wrap it in square brackets:

my $files_to_send = [ map {"$stage/$_.pdf"} @{$self->_getGood()} ];
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The reason you get Can't use string ("6") as an ARRAY ref is that the array returned by map, in a scalar context, is turned into the cardinality of the array, so $files_to_send gets the value 6.

I even went so far as to declare $files_to_send = []; like so, and I still get the same results.

This is the part that I don't understand. Setting $files_to_send to a reference to an empty array should give you no output in the code you have above. It really gives the same error, or it fails a different way?