That doesn't answer my question as to what the output for the input I showed should look like.

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

my @data;
my %tags;
my $cur_tag;
while (<DATA>) {
    chomp;
    if (/^BEGIN_TAG/)
        { push @data, {} }
    elsif ( my ($tag,$line) = /^(X\S+)\s+(.+)$/ ) {
        $data[-1]{$tag} .= " NewRec " if exists $data[-1]{$tag};
        $data[-1]{$tag} .= $line;
        $tags{$tag}++;
        $cur_tag = $tag;
    }
    elsif ($cur_tag) {
        $data[-1]{$cur_tag} .= " NewLine ".$_;
    }
}

use Text::CSV;
my $csv = Text::CSV->new({binary=>1,auto_diag=>2,eol=>"\n",
    sep_char=>";",always_quote=>1,blank_is_undef=>1});
$csv->print(select, [sort keys %tags]);
for my $row (@data) {
    my @vals = map { $row->{$_} } sort keys %tags;
    $csv->print(select, \@vals);
}

__DATA__
BEGIN_TAG
X1 a b
X2 one two
X4 foo
X1 c d
X3 test
BEGIN_TAG
X1 e f
g h
X2 three four
X4 bar
quz baz
X2 five
six
[download]

Output

"X1";"X2";"X3";"X4"
"a b NewRec c d";"one two";"test";"foo"
"e f NewLine g h";"three four NewRec five NewLine six";;"bar NewLine q
+uz baz"
[download]