If I'm understanding you correctly, you want 10.pl to look in additional lib directories, which are not set by default when 10.pl is being run via system from within 8.pl (?)

If so, you could set PERL5LIB prior to calling 10.pl, or call the perl interpreter explicitly, passing it the additional lib directories via one or more -I options (see perlrun).

Update: actually, reading the other replies, it appears you don't want the 5.8 directories on the lib list for 10.pl ... in which case try unsetting PERL5LIB.  PERL5LIB might be one reason for them being there.