DB<3> ("a"x 1e6) =~ / (. (?: (?1) | ) ) /x; say length $1
1000000

  DB<4> ("a"x 1e7) =~ /(.*)/; say length $1
10000000

  DB<5> ("a"x 1e7) =~ / (. (?: (?1) | ) ) /x; say length $1
Killed