The last time suidperl appeared in Slackware was in 8.1; the version of perl was 5.6.1.
/usr/bin/suidperl -> sperl5.6.1
You can try uninstalling your current verison of perl and installing the older slackware
package; download the following and use pkgtool to remove the old and install the new:
Im not sure if this will break any dependencies though! I'll try it myself.
I dont know about the security implications of suidperl, but it might be a good
idea to not allow your webmail to post to root.