Leopard comes with Perl 5.8.8 with threads enabled.
Characteristics of this binary (from libperl):
Compile-time options: MULTIPLICITY PERL_IMPLICIT_CONTEXT
PERL_MALLOC_WRAP USE_ITHREADS USE_LARGE_FILES
USE_PERLIO USE_REENTRANT_API
Which means that, occasionally, stuff like this happens:
make: *** No rule to make target `/System/Library/Perl/5.8.8/darwin-thread-multi-2level/CORE/config.h', needed by `Makefile'. Stop.