sub perl_thinks_it_is_number {
    eval { 
        use warnings FATAL => 'numeric';
        no warnings 'void';
        0+shift;
        1;
    }
}

print "'foo' isn't a number\n" 
    unless perl_thinks_it_is_number 'foo';

print "'17' is a number\n"
    if perl_thinks_it_is_number '17';