$ perl -E " $_ = 123; $f = '(?{ warn pos })';  m/\d$f/; "
Eval-group not allowed at runtime, use re 'eval' in regex m/\d(?{ warn pos })/ at -e line 1.

$ perl -E " $_ = 123; $f = '(?{ warn pos })';  $g = qr/\d$f/; m/$g/"
Eval-group not allowed at runtime, use re 'eval' in regex m/\d(?{ warn pos })/ at -e line 1.