Your code does not hang on my machine, with perl-5.10.0.
(?:[^{}]+?|\{(??{$parens})\})*

Such a regex with nested quantifiers is potentially dangerous, because the outer quantifier can force the inner quantifier to backtrack. Use a non-backtracking group (?>...)* instead. (Oh, and why are you using non-greed +? on the [^{}] group? I see no reason for that).

In reply to Re: balanced parens regexp hangs by moritz
in thread balanced parens regexp hangs - solved by grizzley

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