#!/usr/bin/perl

use strict;
use warnings;

# NOTE: The @sourcedata array is the representation of 
# the data as if it were read from a file by:
#
#   open(FH, $sourcefilename) || die ...
#   my @sourcedata = <FH>;
#   close FH;
#
# Since the source file is huge, I need to process the
# file line by line

# NOTE: I updated this array to reflect an array of lines
my @sourcedata = (
"\n",
"1\n",
"\n",
"\n",
"b\n",
"\n",
"1\n",
"\n",
"\n",
"\n",
"\n",
"_____ 2\n",
"\n",
"\n",
"\n"
);

# The desired result of processing a small data sample:
my @desiredoutput = qq(

1

b

1
_____ 2    # NOTE: preceding newlines have been collapsed



);

##</code><code>##


my @buffer = ();
my $length = scalar @sourcedata;

for (my $I = 0; $I < $length; $I++) {
  my $line = $sourcedata[$I];
  if ($line =~ /^1$/) {
    push(@buffer, $line);
    $I++;
    $line = $sourcedata[$I];

    # Here's the loop I can't seem to execute:
    while ($line =~ /^\n$/ && $I != $length) {
      print "Buffering...\n";
      push(@buffer, $line);
      $I++;
      last if $I == $length;
      $line = $sourcedata[$I];
    }
    if ($line =~ /_____ 2/) {
      # print only the first and last items in the buffer,
      # effectively removing the empty lines
      print shift(@buffer), pop(@buffer);
      print $line;
    }
    else {
      print join(@buffer);
    }
  }
  else {
    print $line;
  }
  @buffer = ();
}