__DATA__
line1=10
line2=30
line3= I am a monk
ine4=false
line5=20
line6=42
line7= But I haven't read perlretut and I should have!
line8=true
line8=34
line9=67
line10=Coder
line11=false

##</code><code>##

line12=00
line2=11
line3=
line4=questionable

##</code><code>##

my @outfile = (
	"line1=10",
	"line2=30",
	"line3= I am a monk",
	"line4=false",
	"line5=20",
	"line6=42",
	"line7= But I haven't read perlretut and I should have!",
	"line8=true",
	"line8=34",
	"line9=67",
	"line10=Coder",
	"line11=false",
);

my @quads;
while (@outfile) {
my $line;
	for (1..4) {              # presumption: each data set is made up of 4 consecutive lines *
		my $line_from_outfile = shift (@outfile);
		$line_from_outfile =~ /line\d+=(.*)/;
		$line .=  $1 . "/" ;
	}
push @quads, $line;
$line = "";
}

for my $quad(@quads) {
	print $quad . "\n";  # to console; writing to file "final.txt" is left as an exercise for OP
}