Maybe it is working correctly, but something is pointing the module search to your 5.18 installation? Do you have PERL5LIB set in your environment?

What happens if you use the full path to pp? And specify the full path to perl?

/Users/rl/perl5/perlbrew/perls/perl-5.16.3/bin/perl /Users/rl/perl5/pe
+rlbrew/perls/perl-5.16.3/bin/pp.pl -M Tk -o TEST2 TKbasic.pl
[download]

Another thing to check is to use scandeps.pl to see where pp is looking for modules.

scandeps -B -V TKbasic.pl

Or even pp with the verbose option (use -vvv for maximum verbosity).

pp -v -M Tk -o TEST2 TKbasic.pl
[download]