Luckily the portability question can be answered rather easily for the more often used modules, by looking at Slaven Rezić's CPAN Tester Matrix, here an example view for DBI.

If it's red on all but one platform, chances are that it's not very portable. If it's mostly green (as in the example above) you don't have to worry too much.

Oh, and it's linked from the distribution page on search.cpan.org, for example from DBI.