$ perl -E  'my $re = qr{ PAT }msx; say $re; $re = qr{ $re }msx; say $re; $re = qr{ $re }msx; say $re; say "..."'
(?^umsx: PAT )
(?^umsx: (?^umsx: PAT ) )
(?^umsx: (?^umsx: (?^umsx: PAT ) ) )
...