I need to share how I solved a similar problem with Package::Stash. Active Perl 5.10.1 Win32, mod_perl 2.0.4. It's a webapp using Template::Toolkit to render the user interface. I have the same "Free to wrong pool" message in Apache's error log.

For example: Free to wrong pool 2248d88 not 2264038 at X:/Path_to_Perl_site_lib/Package/Stash.pm line 40.. Each one triggered an Apache restart like this: [notice] Parent: child process exited with status 255 -- Restarting.

After a lot of research on Google and reading obscure posts about how Apache, mod_perl and XS manage threads on Win32, I went to Stash.pm and noticed it chooses between a Pure Perl and an XS implementation around line 24:

        for my $impl ('XS', 'PP') {
            if (eval "require Package::Stash::$impl; 1;") {
                $IMPLEMENTATION = $impl;
                last;
            }
            else {
                $err .= $@;
            }
        }
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I simply switched 'XS' and 'PP' around in this for loop and now it chooses the Pure Perl implementation first. Problem solved. I restarted Apache and the error log don't show the "Free to wrong pool" message anymore.

Thanks to the monks who reported this problem here and helped me solve a similar issue!