Shouldn't putting them all on one line work? Like
in reply to Re^2: CPAN shell - passing params for module installation
in thread CPAN shell - passing params for module installation
o conf makepl_arg LIB=/path/to/my/distros/perllib PREFIX=--perl_only
Update: Or edit ~/.cpan/CPAN/MyConfig.pm directly and set the things you need from there.
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