(?{ code })

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Eval-group not allowed at runtime, use re 'eval' in regex ...

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For reasons of security, this construct is
forbidden if the regular expression involves
run-time interpolation of variables, unless the
perilous ""use re 'eval'"" pragma has been used
(see re), or the variables contain results of
""qr//"" operator (see ""qr/STRING/imosx"" in
perlop).

This restriction is because of the wide-spread and
remarkably convenient custom of using run-time
determined strings as patterns. For example:

$re = <>;
chomp $re;
$string =~ /$re/;