Not being able to open a file to calculate its MD5 digest means that we can't process that file for coverage data. This is a warning rather than an error because processing of the other files can continue. Adding the word "Warning" makes the status more clear.
Unfortunately we can't get all the way there yet. Perlbrew isn't building early versions of perl that we need yet. perl-5.6. and 5.8.0 at least weren't building cleanly for me and, although it doesn't seem to advertise it, cpanm seems to have a minimum version requirement somewhat higher then this too. When we eventually drop support for 5.6.x and, at least, early 5.8.x we can probably come back to this. In the meantime we'll comtinue with buildperls.
And a bit about the requirements to build Devel::Cover. Based on a patch from Kirk Kimmel.
Forking on Windows requires ithreads which precludes Devel::Cover. Problem noted by Christian Walde.
Problem noted by Daisuke Maki. I'm not sure how that can happen so let's be defensive.