Problem with Perl warnings and regex Use of uninitialized value in regexp compilation

I am calling the following script from my main Perl program开发者_JAVA技巧. The script takes a process name and returns its PID. The script is included in my main perl code by using the require key word:

require "getPid.pl";

and called using:

&pidGetter($processName);

getPid.pl is:

#!/usr/bin/perl -w

use strict;
use warnings;

use Proc::ProcessTable;

pidGetter($ARGV[0]);

sub pidGetter
{
   my $ret="PROCESS ID NOT FOUND\n";
   my $t = new Proc::ProcessTable;
   my $procName = $_[0];

   foreach my $p (@{$t->table})
   {
      if ($p->fname =~ /$procName/)
      {
         $ret = $p->pid;
      }   
   }

   return $ret;
}          

However, when the script is called, I get the following warning:

Use of uninitialized value $procName in regexp compilation at getPid.pl line 19

The rest of the script seems to function fine. It is my understanding that $procName is initialized by $procName = &_[0];

I have put in print statements to debug, and $procName does return a value, so it is initialised. Does anyone know why I am getting these warnings?

require "getPid.pl"; evaluates the code contained in getPid.pl. So you actually call the pidGetter() function twice: in the require'd script and in the main script. As $ARGV[0] is undef inside the require'd script, you get the warning.