I'm curious why you would add this in a script that is supposed to run under older versions

It's all to do with the FileHandle::Fmode test suite. The module should work ok with perl 5.00505, but offers additional features for later builds of perl. The test suite needs to test the additional features on recent builds of perl - but to not test those features on ancient builds of perl (where such tests break perl).

Cheers,
Rob