Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

Assuming that you don't care about IEEE notations like ``NaN'' or ``Infinity'', you probably just want to use a regular expression.

   warn "has nondigits"        if     /\D/;
    warn "not a natural number" unless /^\d+$/;             # rejects -3
    warn "not an integer"       unless /^-?\d+$/;           # rejects +3
   warn "not an integer"       unless /^[+-]?\d+$/;
   warn "not a decimal number" unless /^-?\d+\.?\d*$/;  # rejects .2
   warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
   warn "not a C float"
       unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

If you're on a POSIX system, Perl's supports the POSIX::strtod function. 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 } 

Or you could check out http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz instead. The POSIX module (part of the standard Perl distribution) provides the strtol and strtod for converting strings to double and longs, respectively.