If you don't want to clobber the $testString contents

Both:

    my $l_count = $testString =~ tr/(//;
    my $r_count = $testString =~ tr/)//;
[download]

and:

    my $l_count = ($testString =~ tr/(/(/);
    my $r_count = ($testString =~ tr/)/)/);
[download]

do exactly the same thing.    Neither of which "clobbers" the string.

not sure if that is returning in a list context

The left-hand side of the assignment determines context, in this case scalar, so the parentheses are irrelevant.