It would be trivial to write a module which works on SunOS, but on Linux (just rely on some OS specific feature which is not available in Linux). Since there is no "CPAN police" catching those programmers who upload modules with this type of compatibility problems, and since the content of CPAN is always subject to change, your question is principally not answerable.

OTOH, since nobody introduces such incompatibilities just for fun, chances are very high that a certain module works on both platforms.