perlio probs

Does anyone have any experience of a bug I've encountered in 5.8.7 only 
whereby occasionally (which is what makes it hard to report) this type 
of code:

$fh = IO::File->new("< input.dat") || die;
binmode $fh;
    # lots of code, even to different package

$fh->read($dat, $num_bytes);

does utf-8 conversion on $dat !!

It's only occasionally (for example in Font::TTF on about 20% of a 
collection of fonts, the reading process scrambles getting the name of a 
font, but only on 5.8.7, works fine on all other versions of perl).

TIA,
Martin