I built mod_perl from ports, but I use the 5.8.0 from ports.

Looking at the Makefile, it looks like you need to override the defaults of PERL5, PERL_VER and PERL_VERSION. A command line like

make PERL5=/usr/local/bin/perl PERL_VER=5.8.0 PERL_VERSION=5.8.0

should do the trick (but no promises ;). Be sure to use the same arguments for make install. If all else fails, build it by hand.

HTH...

Update: s/PERL/PERL5/ in command line

Update 2: Note that there should not be a -D, ie: not '-DPERL5=...' but 'PERL5=...'