(Just updated the description: with the new perl5 port, corelist is installed by perl5. The problem still persists, no matter whether perl 5.8, 5.10 or 5.12 are installed.)
Do I get this right? At the moment, trying to make things compatible with perl 5.8 breaks them for *all* versions of perl. Not good.
IOW, if perl 5.12 is installed (as is the default now), the dependency for p5-module-load-conditional is satisfied by simply having the perl5 port installed. p5-module-load-conditional really only requires p5-module-corelist (which, however, will still conflict with perl5) if perl 5.8 is selected.
So would a possible short-term intervention be to change the p5-module-load-conditional port to only depend on p5-module-corelist in a variant called perl5_8? Things would still be broken, but at least not for everybody, but only for those still requiring perl 5.8.
In the long term, I agree with ricci: Short of dropping support for perl 5.8, the "right" solution would be to remove corelist from the perl5 port and to add a dependency on p5-module-corelist to all ports needing it. Identifying those might be a daunting task.