$ perl -E " $_ = 123; m/\d(?{ warn pos })/"
1 at (re_eval 5) line 1.

$ perl -E " no re qw/eval/; $_ = 123; m/\d(?{ warn pos })/"
1 at (re_eval 5) line 1.

$ perl -E " $_ = 123; $f = qr/\d(?{ warn pos })/; m/$f/;"
1 at (re_eval 5) line 1.

$ perl -E " no re qw/eval/; $_ = 123; $f = qr/\d(?{ warn pos })/; m/$f/;"
1 at (re_eval 5) line 1.