Contrary to Spolsky who said "perl is pretty much forgotten"... perl is very much alive.
You have not indicated which version of FreeBSD you have installed... But assuming that you have installed a min of FreeBSD 7.1 then you failed to install the threads libraries or the thread libs are not in the library path.
NOTE that libc is a general 'c' library and that libc_r is the thread-safe version of that file. Some of the 2005 google results suggest that libc_r has been deprecated.
With that in mind it is likely that you are in the middle of a "revision collision". Sadly FreeBSD does not provide the thread safe version of perl natively, however, they do package perl 5.10 and several later revisions. I found this link to be useful:
http://www.nabble.com/How---where-to-get-a-Perl-that-has-threads--td22270858.html
you might also want to try installing the p5-fork library which is included with FreeBSD
http://www.nabble.com/How---where-to-get-a-Perl-that-has-threads--td22270858.html