sub AsNumber {
    my ($s) = @_;
    return undef unless defined( $s );

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

print AsNumber(123), "\n";
print AsNumber("123"), "\n";
print AsNumber("abc"), "\n";
print AsNumber(1.2e-10), "\n";
print AsNumber(1.23), "\n";
print AsNumber("1.2a"), "\n";