Since your local $SIG{__WARN__}= sub {}; doesn't suppress that one warning, I can only conclude that the warning is happening before that line is run or after the enclosing scope is left (likely it is happening when you use that module, though I uncharacteristically haven't looked at the source code for that module to determine if that line number gets run at use time -- use statements get run when they are compiled, which will be before your __WARN__ override gets run). So switch it to this:

BEGIN {
    $SIG{__WARN__}= sub { };
}
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and I expect you can adjust it after that to only suppress the warnings that you want suppressed.

There is also #/usr/bin/perl -X which just disables all warnings.