Altering URIs with regular expressions can get weird and break pretty fast. Something like this, using the modules I recommended earlier, is better-

perl -MCGI=self_url -le 'print self_url' "o=hai;i=can;haz=cheezeburger
+"
http://localhost?o=hai;i=can;haz=cheezeburger

perl -MCGI=self_url -MURI -MURI::QueryParam -le \
  '$u = URI->new(self_url); $u->query_param(haz => "tacoz"); print $u'
+ \
  "o=hai;i=can;haz=cheezeburger"
http://localhost?o=hai;i=can;haz=tacoz
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