How to derefence a copy of a STDIN filehandle?

Your $filehandle should already be open - you had opened FILECOPY, taken a reference and put it in $FH_ref, which is $filehandle. If you want to re-open it again use the <& argument in open or just start reading from it right away.

If I understand correctly, you want the 3-arg equivalent of

open my $fh, '<&STDIN'

That would be

open my $fh, '<&', $filehandle