$ perl -le " qr{\d****} "
Nested quantifiers in regex; marked by <-- HERE in m/\d** <-- HERE **/ at -e line 1.

$ perl -le " qr{(?#an error follows)****} "
Quantifier follows nothing in regex; marked by <-- HERE in m/(?#an error follows)* <-- HERE ***/ at -e line 1.

$ perl -le " qr{?} "
Quantifier follows nothing in regex; marked by <-- HERE in m/? <-- HERE / at -e line 1.