use warnings;
no warnings "redefine";

##</code><code>##

{
    no warnings "redefine";
    sub that_must_be_redefined {
        ...
    }
}

##</code><code>##

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