$ man perlre

    Starting in Perl 5.14, a "^" (caret or circumflex accent)
    immediately after the "?" is a shorthand equivalent to
    "d-imnsx".  Any positive flags (except "d") may follow
    the caret, so

        (?^x:foo)

    is equivalent to

        (?x-imns:foo)

    The caret tells Perl that this cluster doesn't inherit the
    flags of any surrounding pattern, but uses the system
    defaults ("d-imnsx"), modified by any flags specified.