#!/usr/bin/perl
use strict; 
use warnings;

my %data;
my $cur_tag;
open (FILE, '<', $ARGV[0]) or die "Could not open file: $!";
while (<FILE>) {
    chomp;
    if ( my ($tag,$line) = /^(X\S+)\s+(.+)$/ ) {
        $data{$tag} .= "\n" if exists $data{$tag};
        $data{$tag} .= $line;
        $cur_tag = $tag;
    }
    elsif ($cur_tag && !/^BEGIN_TAG/) {
        $data{$cur_tag} .= " NEWLINE ".$_;
    }
}

for my $tag (sort keys %data) {
    print "$tag: $data{$tag}\n";
}

##</code><code>##

BEGIN_TAG
X1 test test test
X2 no no no no
X3 yes yes yes
BEGIN_TAG
X1 test test test
tes test test
X2 no no no no no 
nono non no no
 nononono no no  no
X3 hi hi hi hi hi hi 
hi hi hi hi 
hi hi

##</code><code>##

X1: test test test
test test test NEWLINE tes test test
X2: no no no no
no no no no no  NEWLINE nono non no no NEWLINE  nononono no no  no
X3: yes yes yes
hi hi hi hi hi hi  NEWLINE hi hi hi hi  NEWLINE hi hi