OK, assuming you have sorted your records by company, you can have something like this:

my $company = "unlikelyname";
my $OUT;
while (<$file>) {
      my @f = split '\s+', $_;
      if ($f[0] !~ /$company/) {
          close $OUT if defined $OUT;     
          open $OUT, ">", "$f[0].txt" or die "blabla $!";
          $company = $f[0];
      }
      my @ymd1 = split ',',$f[4] //= $prev_mth_end;
      my @ymd2 = split ',',$f[5] //= $prev_mth_end;
      # ...

     printf OUT $pfmt, @f[0..3], fmt_mdy($f[4]), fmt_mdy($f[5]), $diff
+, $free, $waive, fmt_curr($cost), $stg_chg, $sw_chg, $f[6], fmt_curr(
+$cartot), $comment;
}
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I don't have data and can't test in detail, there may be some errors here and there, but I am fairly sure the basic idea works and it is very simple: you change company? Just close the previous file, open a new file with the same filehandler but a new name, so that you can always write to $OUT (and nonetheless to the right file), which will be at any time associated with the right file.