Kill Perl Golf:

$ perl -e '/(?{<})/'
panic: top_env
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If you swap the less then sign, it's just a syntax error:

$ perl -e '/(?{>})/'
syntax error at (re_eval 1) line 1, near ">"
Compilation failed in regexp at -e line 1.
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(Reason: anything that might be the start of a quoted construct inside (?{}), but isn't followed by one panics perl. (?{'}), (?{"}), (?{`}), (?{<}), (?{/}), (?{?}), (?{q}), (?{m}) and (?{s}) all panic perl. Other single charactes seem to do ok (although I didn't check all of them).