$ perl -Mre=debug -e '/\d++/'
Compiling REx "\d++"
synthetic stclass "ANYOF[0-9][{unicode_all}]".
Final program:
   1: SUSPEND (7)
   3:   PLUS (5)
   4:     DIGIT (0)
   5:   SUCCEED (0)
   6: TAIL (7)
   7: END (0)
stclass ANYOF[0-9][{unicode_all}] minlen 1 
Freeing REx: "\d++"
$ 
$ 
$ perl -Mre=debug -e '/\d+++/'
Compiling REx "\d+++"
Nested quantifiers in regex; marked by <-- HERE in m/\d+++ <-- HERE / at -e line 1.
$