perl: re-enable the postPatch hook on Darwin

The problem was that sed(1) variants other than GNU sed require a backup suffix
to the -i option.
perl: revert "re-enable the postPatch hook on Darwin"

This reverts commit 1778200519a18ae00751ab3015d0e665373c32f6.
Apparently, the sed -i syntax is different from what I thought, because
now the build fails on Darwin, saying:

  sed: -i.bak: No such file or directory

See http://hydra.nixos.org/build/5829944/nixlog/1/raw for more details.