$ perl -Mre=debug -le "print qr/\d/"
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
(?^:\d)
Freeing REx: "\d"

$ perl -Mre=debug -le "print qr/\d/u"
Compiling REx "\d"
Final program:
   1: DIGIT (2)
   2: END (0)
stclass DIGIT minlen 1
(?^u:\d)
Freeing REx: "\d"

$ perl -Mre=debug -le "print qr/\d/a"
Compiling REx "\d"
Final program:
   1: DIGITA (2)
   2: END (0)
stclass DIGITA minlen 1
(?^a:\d)
Freeing REx: "\d"

$ perl -Mre=debug -le "print qr/\d/aa"
Compiling REx "\d"
Final program:
   1: DIGITA (2)
   2: END (0)
stclass DIGITA minlen 1
(?^aa:\d)
Freeing REx: "\d"