Adding threads support to perl built by MacPorts

I had installed perl5.12 from MacPorts. sudo port installed listed

perl5                          @5.12.3         lang/perl5
perl5.12                       @5.12.3         lang/perl5.12
perl5.8                        @5.8.9          lang/perl5.8

To enable threading support, I used

sudo port upgrade --enforce-variants perl5.12 +threads