is there that much of a difference between a strictly Perl-based use lib and setting PERL5LIB in mod_perl?

Mostly, that perl only reads $ENV{PERL5LIB} when it's started:

use lib '/some/dir';
use Module;
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will work as expected, while

BEGIN {
  $ENV{PERL5LIB} = "/some/dir:$ENV{PERL5LIB}";
}
use Module;
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will not, if Module.pm is in /some/dir.

Either use lib or BEGIN { unshift @INC }, but you have to do both from perl; you can't use PERL5LIB, as perl is already running before apache starts reading the config files.