BEGIN {
    my $debug = 0;
    unless ($debug) {
        $INC {"warnings.pm"} = 1;
        no strict 'refs';
        *{"warnings::import"} = sub {1;};
    }
}

use warnings;
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If you can live with just one category enabled if debugging is off, you could do:

BEGIN {
    $::DEBUG = 1;
}
use warnings $::DEBUG ? "all" : "io";  # Or some other category.
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