Yup, what davido said (and I said), see http://perl5.git.perl.org/perl.git/blobdiff/cc08d69f51c86cbd5787050beb51daf9bfa5c373..10a4597703f4b044d3f968bf3923644ea2387f5c:/dist/threads/lib/threads.pm it says discouraged

+=head1 WARNING
+
+The "interpreter-based threads" provided by Perl are not the fast, li
+ghtweight
+system for multitasking that one might expect or hope for.  Threads a
+re
+implemented in a way that make them easy to misuse.  Few people know 
+how to
+use them correctly or will be able to provide help.
+
+The use of interpreter-based threads in perl is officially
+L<discouraged|perlpolicy/discouraged>.
+
[download]

Which is just FUD, recipes instead of warnings