To clarify, (?{}) constructs can be used without the use re 'eval' pragma, but only if the regex contains no elements interpolated from variables. Perhaps an example is called for:

#fine
/a (.*) a(.)*(?{push @substrings, $^N}) (?!)/


#not fine: we get part of the regex body from a variable
/$regex(?{push @substrings, $^N}) (?!)/
 ^^^^^^ this variable interpolation requires that you use re 'eval'
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Type perldoc re at your command prompt for more info.