driftingprogrammer SO question

#!/usr/bin/perl

use warnings;
use strict;

open (XIN, '<', 'driftingprog-in.xml') or die "$!\n";
open (XOUT, '>', 'driftingprog-out.xml') or die "$!\n";

my %content;

## stores 3 lines from file, used by second loop
my @block;

while (<XIN>) {
  if (/<content ID="(.*?)">(.*?)<\/content>/) {
    my ($id, $text) = ($1, $2);
    $content{$id} = $text;
  } elsif (/<text>/) {
    ## keep this line for next loop
    push @block, $_;
    ## when we start seeing <text> tags, go to next loop for these
    last;
  }
  print XOUT $_;
}

while (1) {
  ## read up to 3 lines into @block
  for (scalar(@block)+1..3) { my $l = <XIN>; last if (!defined $l); push @block, $l; }
  ## if we've read nothing, we are at EOF
  last if (scalar(@block) == 0);

  my $concat = join '', @block;
  if ( ($concat =~ s/(<text>\s*)<reference value="#(.*?)" \/>(\s*<\/text>)/$1.$content{$2}.$3/es) > 0) {
      print XOUT $concat;
      @block = ();
  } else {
      print XOUT shift @block;
  }
}

close XOUT;
close XIN;