use warnings;
use strict;

my $data = do { open my $fh, '<', 'sample.txt' or die $!;
	local $/; <$fh> };  # slurp whole file into memory

my $regex = qr{ 
	^ \s*APP>\sSTART_PARTU_8X8 \n
	^ \s*APP>\sPARTITION_SPLIT \n
	(.*?)
	^ \s*APP>\sEND_PARTU_8X8 $
	}msx;

if ($data=~$regex) {
	print $1;
}