$file='
level 1
section1
section2
level 2
section3
section4
level 2
';

while ($file=~s#(level [0-9].+?)(level [0-9])#$2#s)
{
	$a=$1;

	@b=$a=~m#(section[0-9])#g;
	push (@array, [@b])
}