At least the plus has a special meaning in a regex, therefore you should quote your variables by putting an \Q in front of them (if you want to match the string literally):

sub counttags()
{
   $stag=0;
   $etag=0;
   while ($_ =~ /<\Q$_[0]>/g) {$stag++}
   while ($_ =~ /<\/\Q$_[0]>/g) {$etag++}
   if($stag != $etag)
   {
      print "Number of Start and End tags for element $_[0] does not m
+atch";
   }
}
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