I don't know, but here's some numbers to ponder.
The initial release of 5.6 was on 2000-03-22.
The last release of 5.6 was on 2001-04-08.
(Not counting a later release which only added support for more compilers.)
Perl 5.6 is 7 years old, and hasn't had a release in 6 years.
Hopefully, just about everyone is using at least 5.6 by now.

In case you are curious:
The initial release of 5.8 was on 2002-07-19.
The latest release of 5.8 was on 2006-01-31.

Information taken from Module::CoreList.