[CentOS] Multiple versions of software

I have a user who wants a version of Perl built with -Duse64bitint
that is accessable as perl64.  I've pulled down the src.rpm and I'm
going through the perl.spec file looking to see what all needs to be
changed.  So, before I really screw up my system, I thought I would
ask if anyone here has any particular advice.  Help?

-- Matt