First, by using qq(), your @sourcedata array is only 1 element. From what I can tell, you need to start buffering when you see 1 and if you see 1 again, just output the buffer. If you see ______2, trim then output the buffer. You'll probably want to empty the buffer after the loop too. 

#!/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: newlines are significant
my $sourcedata = qq(

1

b

1




_____ 2



);

my @sourcedata = split( /\n/, $sourcedata );

my $buffer = [];
my $in_1   = 0;

foreach my $line ( @sourcedata ) {
  if( $line =~ /^1$/ ) {
    if( $in_1 ) {
      # all ready in 1 so just output buffer;
      out_buffer( $buffer );
      $buffer = [];
    } else {
      $in_1 = 1;
    }
  }
  if( $line =~ /^_____ 2$/ && $in_1 ) {
    trim_buffer( $buffer );
    out_buffer( $buffer );
    $buffer = [];
    $in_1 = 0;
  }

  if( $in_1 ) {
    push( @$buffer, $line );
  } else {
    print $line,"\n";
  }
}

# empty buffer
out_buffer( $buffer );

# subs

sub out_buffer {
  my $buffer = shift;
  foreach my $b ( @$buffer ) {
    print $b, "\n";
  }
}

sub trim_buffer {
  my $buffer = shift;
  my $len    = scalar( @$buffer ) - 1;

  for( my $i = $len; $i >= 0; $i-- ) {
    if( ! $buffer->[$i] ) {
      splice( @$buffer, $i, 1 );
    }
  }
}

[download]
-derby

In reply to Re: Implementing a parsing rule in a huge data file by derby
in thread Implementing a parsing rule in a huge data file by thezip

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