Here's what I set in my CPAN config:

LIB=/var/webeng/lib/perl
PREFIX=/var/webeng/lib/perl
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and this is what I export before installing new modules (so CPAN knows whats already installed):

export PERL5LIB=/var/webeng/lib/perl
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And finally, this is what I add to my code to use the "private" modules:

use lib qw(/var/webeng/lib/perl);
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-Nitrox