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PERL Language is older than PHP, that was developed as a successor of this first language. The consequence is that PERL forum software will be often "old" software, that were started at a moment PHP was either not existing or considered as a correct alternative.
Using an old forum software is however not especially a bad idea, because 10 years of experience and bug fixing is generally a good insurance of having a secure and polished software, used by thousand people before you. This, of course, does not take the style into account.