I may be mistaken, but this does not detect a case like

"}a{"

, or does it?

how about

sub balanced {
    (my $s = shift) =~ s/[^()]//g;
    while ( $s =~s /\(\)// ){};
    return $s ? 0 : 1;
}
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It does not use qr and is slower, I would think, but reports '}{' as incorrect