If you just localize it with local $SIG{__WARN__} = ... then it'll clean itself up without any work required by you. Further... if you really wanted to you could assign your own sub and filter out just the warnings you want to ignore.

local $SIG{__WARN__} = sub {
    warn $_[0] unless $_[0] =~ /^Subroutine .+ redefined/;
}
[download]