#!/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";
}