my %hash;

open DATA,"<filename.txt" || die "Couldn't open $!";
## we'll process each line individually instead of
## slurping it into an array. That way any size file
## will be usable
while (<DATA>) {
  chomp;
  my @vars=split /,/,$_;
  ## ignore it if no first value
  next unless defined $vars[0];
  ## I will also assume your regex were to remove all
  ## leading and trailing spaces. If that is true,
  ## you did it incorrectly
  map { s/^\s*//; s/\s*$// } @vars;
  ## and store it based on the 3rd var
  push @{$hash{$vars[2]}},@vars[0,1,3];
}
close DATA;