I have ActiveState Perl 5.8, Build 805 on my system at work, and according to the thread documentation:

Perl 5.6 introduced something called interpreter threads. Interpreter 
+threads are different from ``5005threads'' (the thread model of Perl 
+5.005) by creating a new perl interpreter per thread and not sharing 
+any data or state between threads by default.

Prior to perl 5.8 this has only been available to people embedding per
+l and for emulating fork() on windows.
[download]

TStanley
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