I've wondered the same thing. I tend to do something like this in my makefile:

SOURCE_DIR = `perl -e 'use Config; print $$Config{sitelib}'`/foo
#... e.g., /usr/local/lib/perl5/site_perl/5.8.0/foo

install:
    $(INSTALL_PROGRAM) -d $(SOURCE_DIR)
    $(INSTALL_PROGRAM) $(PERL_SOURCES) $(SOURCE_DIR)
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Not sure if this is the best thing to do on all systems -- I run FreeBSD.