perl -le "print for @INC"
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will tell you the module search order for your perl executable.

I guess that you will have to maintain the dependencies you introduce yourself, or at least take some precautions in your RPMs. I don't know if RPM has a "provides" rule so you can say:

foo-2.0:
    #  this is your added package

perl-5.10.0:
    provides foo-1.0
    requires foo-2.0 # so that RPM also pulls in foo-2.0

perl-5.10.1:
    provides foo-3.0 # and RPM will automatically remove the foo-2.0 p
+ackage
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But again, I think I wouldn't try to fudge around with the Perl distribution itself but create an enclosing package that pulls in the required separate RPMs:

foo-2.0-meta:
    requires foo-2.0
    requires perl-5.10.0
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That way, once you upgrade to foo-3.0, it could relax the requirement on the foo-2.0 package and RPM could remove it from the system.