As a side-note, in general, don't use $@ to test whether an eval succeeded or failed. It's quite easy to come up with examples where an eval failed, but $@ is false, or where an eval succeeded, but $@ is true (for instance, due to actions happening at DESTROY time). Always make sure your block returns true, and test the return value of eval: if it's undefined, the eval failed:

    eval {
        # make conversion failure a fatal error
        use warnings FATAL => qw(numeric);
        # try to use $s as a number
        $number = 0 + $s;
        1;
    } or do {
        my $err = $@ || "Unknown error";
        # exception happened, thus not a number
        $number = undef;
    };
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