Just put the cgi.assign in the host pattern, unless you really really need to have .pl files not be executable outside the boards. Less redundant, and less chance of stuff randomly not working because of a silly typo.
If you map a script to "", it'll execute the script directly with the hashbang line. This is the way I do it because it's more like other servers and (IMO) less prone to odd problems.
So your vhost would look like:
$HTTP["host"] =~ "steamchan\.net$" {
server.document-root = "/var/www/steamchan"
cgi.assign = (".pl" => "")
}Then just chmod +x wakaba.pl and make double sure the cgi scripts have write access to the directories. It "should" work, for varying values of should.