If you mean the parens in the regex, you don't need them using the other way, either. /PAT/g in list context, will imply parens around the whole match, if none are provided in the pattern.

$count = () = $teststring =~ /test/g;
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will just work fine.

This is documented in perlop, see the last sentence in this paragraph:

The "/g" modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern.