I don't have the source here, therefore from the top of my hat: A few month ago I had studied this region in 5.005 for working with filters. I think the opened FH is a glob, which is stored in %INC. The $ of this glob is saved the fileno into a lot of lines earlier (code folding helps).
So I think you must write:open my $x,"<&=".$*{delete $INC{$key}}