Probably I'm misunderstanding something about your question, but summing up is simply done like this:

...
my $sumcredits;
while (my $pimrpt = <PM>) {
...
  $sumcredits+= $credits;
...
}
print "The sum of credits is $sumcredits\n";
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If you need to add credits seperate for each company, change that to

...
my %sumcredits;
while (my $pimrpt = <PM>) {
...
  $sumcredits{$company}+= $credits;
...
}
foreach my $comp (keys %sumcredits) {
  print "The sum of credits for $comp is $sumcredits{$comp}\n";
}
[download]