Close, but no cigar :) HTTP::Request::Common has the detailed docs

[ $local_file_to_open, $filename_to_send, Header => Value... ]
[ undef, $filename_to_send, Header => Value,..., Content => $file_cont
+ents ]
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If $filename_to_send eq "", none is sent, if $filename_to_send is undef, then basename of $local_file_to_open is sent