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$ perl -E 'my $re = qr{(?x: PAT )}; say $re'
(?^u:(?x: PAT ))

(?d-imnsx:(?u-d:(?x: PAT )))

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

(?d-imnsx:(?umsx-d: PAT ))

$ perl -E 'my $re = qr{(?d-imnsx:(?u-d:(?x: PAT )))}'
Regexp modifier "d" may not appear after the "-" in regex; ...
$ perl -E 'my $re = qr{(?d-imnsx:(?umsx-d: PAT ))}'
Regexp modifier "d" may not appear after the "-" in regex; ...

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