I assume that on your part this is one of those not seeing the forest for the trees case. :) I know those too well.

my $clone_warn = \&warnings::warn;

*warnings::warn = sub {
    print "Look ma!\n";
    $clone_warn->(some_filter_function_i_provide @_);
};
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Just take a reference to whatever it was you want to keep..

Update: clone_warn(foo) was an overlooked copypaste remnant.. It is a scalar holding a coderef of course.

Makeshifts last the longest.