For balanced begin/end matching, this might do:

$re = qr{
        (
            (?:
                (?&OPEN)
                (?:
                    (?>(?:.*?(?=(?&OPEN)|(?&CLOSE))))
                    |
                    (?-1)
                )*
                (?&CLOSE)
            )
        )
        (?(DEFINE)
            (?<OPEN>\bBEGIN\b)
            (?<CLOSE>\bEND\b)
        )}x;
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