That's an FAQ actually, as you can find browsing the output of perldoc -q number:

       How do I determine whether a scalar is a number/whole/integer/f
+loat?
                                                                      
+          
       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+)$/) { print "is a decimal num
+ber\n" }
          if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                               { 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 funct
+ion
       "looks_like_number" for determining whether a variable looks li
+ke 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 typ
+es 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 "getn
+um"
       wrapper function for more convenient access.  This function tak
+es a
       string and returns the number it found, or "undef" for input th
+at 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 ins
+tead.
       The POSIX module (part of the standard Perl distribution) provi
+des the
       "strtod" and "strtol" for converting strings to double and long
+s,
       respectively.
[download]

I had your same problem, too :-)