"Use of uninitialized value" error message

[Ho, Tony]

Hi guys
I was wondering if you could help me with the following problem.
 
I am getting the following error message:
 
Use of uninitialized value in string ne at format_imsi_msisdn.pl line 257.

line 257 and beyond consist of the following:
 
if ($result_value1 ne " ") {
          $a= substr($result_value1, 0, 8);
          $b= substr($result_value1, 8, 2);
          $c= substr($result_value1, 10, 2);
          return ($a, $b, $c);
}
else {
          return (" ", " ", " ");
}
 
I declare the variable result_value1 at the begining of the method as
follows
 
my $result_value1 =" ";
 
Any ideas why ?
 
Thanks in advance
Tony