use Scalar::Util;
use warnings;

$string = '5.' . '0' x 30 . '1';;

print $string, "\n";

if(Scalar::Util::looks_like_number($string) &&
  $string > 0 && $string == int($string) ) {
  print "looks like a positive integer\n";
}
else {print "not a positive integer\n"}

__END__
Outputs:
5.0000000000000000000000000000001
looks like a positive integer