I'd just install perl5.8 into a new directory (e.g. d:\perl5.8; take care not to change file assoziations) and add it to %PATH%, but after the old perl (so if you call perl scriptname.pl, you will use perl5.6). If you want to use perl5.8, just use perl5.8.0 scriptname.pl...
If you use CGI-Scripts with Ms Internet Information server, just assign a new file extension (e.g. .pl58) with perl5.8

Best regards,
perl -e "s>>*F>e=>y)\*martinF)stronat)=>print,print v8.8.8.32.11.32"