Copyright question.

If I have modified a work that is released under the GNU GPL or the Perl 
Artistic License, who owns the copyright on the work?  Who owns the 
copyright of the modifications?  How do I indicate copyrights properly?

Thanks,
--
Heath Caldwell
hncaldwell@xxxxxxxxxxxxx, GPG-Key-ID: 0x0967843B
Web Programmer
I&IT, Studio 6 - Cal Poly Pomona