That's exactly what I wanted to know. So, what do you think of my sub?

Sorry, but I don't think it's a good idea. For one, the signal handler will fire on any warning. Scalar::Util's looks_like_number (a core module) calls the internal Perl function that checks if a string looks like a number, so this is just a really convoluted way of calling that function. In your OP, you said "Is there a faster way to test if a variable is a number?", and this is definitely a much slower way to do so - in fact, roughly 38 times slower! As numerous people have said, just use looks_like_number.

use warnings;
use strict;
use Benchmark qw/cmpthese/;
use Scalar::Util qw/looks_like_number/;

sub isNumber {
    @_ or return 0;
    my $N = shift;
    defined $N or return 0;
    my $R = 1;
    {
        local $SIG{__WARN__} = sub { $R = 0; };
        $N = int($N);
    }
    return $R;
}

cmpthese(-2, {
    isNumber => sub {
        isNumber("123") or die;
        isNumber("-5e7") or die;
        isNumber("abc") and die;
        isNumber("") and die;
    },
    looks_like_number => sub {
        looks_like_number("123") or die;
        looks_like_number("-5e7") or die;
        looks_like_number("abc") and die;
        looks_like_number("") and die;
    }
});

__END__

                       Rate          isNumber looks_like_number
isNumber           153840/s                --              -97%
looks_like_number 5991731/s             3795%                --
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