foreach (3,'3',3.0,'3.0',3.1,'3.1') {
    print "$_ is ", is_numeric($_) ? '':'not ', "numeric\n";
}

sub is_numeric {
    ($_[0] & ~ $_[0]) eq "0";
}


$ ./is_numeric.pl 
3 is numeric
3 is not numeric
3 is numeric
3.0 is not numeric
3.1 is numeric
3.1 is not numeric