...

write a program to do it, say utils/fixup_permissions.pl

or edit ExtUtils::Install and maybe something else (meh)

So that they're usable by members of GROUP?

Well, they're still usable by perl :)

update: crossposted to http://stackoverflow.com/questions/17598387/how-do-i-install-a-perl-module-with-specific-permissions, you should cross-link cross-postings for maximum collaboration efficiency