substr help

[Bobby]

Could someone please tell me what's wrong with my use of the substr function 
below? I keep on getting "use of uninitialized value in substr". Thanks. 

$newpsc = substr($PSC,0,$count);

Here's the complete code:

#!/usr/bin/perl
use strict;
use warnings;

my $exclude_psc = 'exclude_psc.txt';
my $current_base  = 'base.txt';
my $output        = 'new_excludepsc.txt';
my %ex_psc;
my $exkeyword;

open EXCLUDE, '<', $exclude_psc or die "Could not open
'$exclude_psc' $!";

while ( <EXCLUDE> ) {
    next if $. == 1;  # exclude header
    chomp;
    my ($excpsc,$keyword) = split /\|/;

    %ex_psc = (exclpsc=>$excpsc,exkeyword =>$keyword );
}
close EXCLUDE;

open BASE, '<', $current_base or die "Could not open '$current_base'
$!";
open OUT,  '>', $output or die "Could not open '$output' $!";

while (<BASE>) {
    my ($base_no, $name, $description, $PSC) = split /\|/;

my $newpsc;
my $wordlength;

my $expsc=$ex_psc{exclpsc};
my $count = $ex_psc{exclpsc}=~ s/([a-z])/$1/gi;
if ($count eq 4){
$newpsc = substr($PSC,0,$count);
  if (uc($ex_psc{exclpsc}) eq uc($newpsc)){
    $_= join '|',$base_no,$ex_psc{exclpsc},$ex_psc{exkeyword};
 print OUT;

}
}
}
close(OUT);
__END__




       
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