use warnings;
no warnings "redefine";
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Just to be on the safe side,

{
    no warnings "redefine";
    sub that_must_be_redefined {
        ...
    }
}
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If wanting to be very very picky...

{
    my $redefined = sub  {
        ...
    }
    no warnings "redefine";
    *that_must_be_redefined = $redefined;
}
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