use strict;
use warnings;

my $start = 'start:';
my $end = 'stop:';

$_ = join '', <DATA>;
@_ = m/\Q$start\E(.*?)\Q$end\E/gs;
print $_[-1];

__DATA__
something start: some text stop: other text here start: again something stop: may be some text here start: and finally stop: and maybe something here