#!/usr/bin/perl
use constant CHUNK => 500 * 1024; # 500 KB
use strict;
use warnings;
use List::Util 'sum';

my $file = $ARGV[0] or die "Usage: $0 <input_file>";
open(my $in_fh, '<', $file) or die "Unable to open '$file' for reading
+: $!";

my ($cnt, @buffer);
while (<$in_fh>) {
    if (is_new_invoice($_)) {
        my $size = sum(map length($_), @buffer);
        if ($size > CHUNK) {
            flush_buffer();
            push @buffer, $_;
        }
        else {
            push @buffer, $_;
        }        
    }
    else {
        $buffer[-1] .= $_;
    }
}
flush_buffer();

sub flush_buffer {
    # variables should probably be passed explicitly
    return if ! @buffer;
    ++$cnt;
    open ($out_fh, '>', "outfile.$cnt") or die "Unable to open 'outfil
+e.$cnt' for writing: $!";
    my $size = 0;
    while (1) {
        my $invoice = shift @buffer;
        last if ! $invoice;
        my $len = length($invoice);

        # If invoice by itself is larger than chunk, write to new file
        if ($len > CHUNK) {
            ++$cnt;
            open ($out_fh, '>', "outfile.$cnt") or die "Unable to open
+ 'outfile.$cnt' for writing: $!";
            print $out_fh $invoice;
            close $out_fh;
            next;
        }

        # If this $invoice puts the current file over the limit, close
+ the current file
        if ($size + $len > CHUNK) {
            close $out_fh;

            # Replace the invoice back in the buffer and return if not
+ end of input
            if (! eof $in_fh) {
                unshift @buffer, $invoice;
            }
            else {
                # Write out whatever is left
                ++$cnt;
                open ($out_fh, '>', "outfile.$cnt") or die "Unable to 
+open 'outfile.$cnt' for writing: $!";
                print $out_fh $invoice;
                close $out_fh;
                
            }
            return;            
        }
        else {
            # Add this invoice to the existing open file
            print $out_fh $invoice;
        }
        last if ! @buffer;        
    }
}

sub is_new_invoice {
    my ($line) = @_;
    return 1 if substr($line, 66, 2) eq '11';
    return 0;
}
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