Since you're assigning to an array reference (my $files_to_send = ...), you should enclose the results of the map in [ ... ]:

my $files_to_send = [ map {"$stage/$_.pdf"} @{$self->_getGood()} ];
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Otherwise, you're assigning to the count of the items in the array (using the array in scalar context).

Oh, and you could also simplify @{$files_to_send} to just @$files_to_send.  In fact, if you don't care about printing newlines, you might make it really simple:

print "@$files_to_send\n";
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