How do I determine whether a scalar is a number/whole/integer/float? Assuming that you don't care about IEEE notations like "NaN" or "Infin- ity", you probably just want to use a regular expression. if (/\D/) { print "has nondigits\n" } if (/^\d+$/) { print "is a whole number\n" } if (/^-?\d+$/) { print "is an integer\n" } if (/^[+-]?\d+$/) { print "is a +/- integer\n" } if (/^-?\d+\.?\d*$/) { print "is a real number\n" } if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) { print "a C float\n" } There are also some commonly used modules for the task. Scalar::Util (distributed with 5.8) provides access to perl's internal function "looks_like_number" for determining whether a variable looks like a number. Data::Types exports functions that validate data types using both the above and other regular expressions. Thirdly, there is "Reg- exp::Common" which has regular expressions to match various types of numbers. Those three modules are available from the CPAN. If you're on a POSIX system, Perl supports the "POSIX::strtod" func- tion. Its semantics are somewhat cumbersome, so here's a "getnum" wrapper function for more convenient access. This function takes a string and returns the number it found, or "undef" for input that isn't a C float. The "is_numeric" function is a front end to "getnum" if you just want to say, ``Is this a float?'' sub getnum { use POSIX qw(strtod); my $str = shift; $str =~ s/^\s+//; $str =~ s/\s+$//; $! = 0; my($num, $unparsed) = strtod($str); if (($str eq '') || ($unparsed != 0) || $!) { return undef; } else { return $num; } } sub is_numeric { defined getnum($_[0]) } Or you could check out the String::Scanf module on the CPAN instead. The POSIX module (part of the standard Perl distribution) provides the "strtod" and "strtol" for converting strings to double and longs, respectively.