They are not written to cater towards concurrency.

As a simple example, let's consider the Perl5 implementation of LWP::UserAgent. It does not support any concurrency mechanism, and once it blocks while reading from a socket, offers no way to relinquish control to elsewhere in the program. If Perl5 had (from the start) had (say) cooperative multithreading, LWP likely would have been written with that in mind already. It would likely also have emitted messages or callbacks about its progress earlier.

Libraries for Perl6 likewise will not support what is not there from the start.

In reply to Re^8: Backend diversity for Rakudo by Corion
in thread Backend diversity for Rakudo by Anonymous Monk

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