use warnings;
use strict;

# maximum possible length of start pattern (and then some).
use constant MAX_START => 10_000;


my $start     = qr{ START         }xms;
my $not_start = qr{ (?! $start) . }xms;
my $stop      = qr{ STOP          }xms;

my $full_pattern = qr{ $start $not_start*? $stop }xms;

my $buffer = '';
my $searching_for_start = 1;

LINE:
while (defined(my $line = <DATA>)) {

    # extracted string WON'T include record seps (usually newlines).
    # comment out if start/stop patterns will not be broken
    # at random across multiple lines.
    chomp $line;

    $buffer .= $line;

    # # looking for start pattern in buffer?
    # if ($searching_for_start) {  # works
    #     # turn flag off if start pattern found.
    #     if ($searching_for_start = $buffer !~ $start) {
    #         # still looking?  limit buffer, search next line.
    #         $buffer = substr $buffer, -MAX_START();
    #         next LINE;
    #         }
    #     }

    # looking for start pattern in buffer and we don't find it?
    if ($searching_for_start &&= $buffer !~ $start) {
        # limit buffer, keep looking.
        $buffer = substr $buffer, -MAX_START();
        next LINE;
        }

    # found start pattern (maybe more than one) in buffer.
    # look for stop pattern (also maybe more than one).
    next LINE unless $buffer =~ $stop;

    # got at least 1 start and stop in buffer.
    # however, buffer may contain STOP then START (inverted order).
    # extract all full patterns, if any.
    my @full_patterns = $buffer =~ m{ $full_pattern }xmsg;

    # keep looking unless we get at least 1 full pattern.
    next LINE unless @full_patterns;

    # clip last full pattern and all before it from buffer.
    $buffer =~ s{ \A .* ($full_pattern) }{}xms;

    # then process extracted pattern(s)...
    process(@full_patterns);

    # ... and go back to searching for start pattern.
    $searching_for_start = 1;

    }

sub process { print "'$_' \n" for @_ }


__DATA__
START
blah
blahblah
START
blah ah
other random stuff
START
blah ha
other random stuff
STOP
asdf
START
STOP
foo
bar
START
fdas
STOP
baz
a START b START c
d e START yes
S
T
OP xxx S
TART xx STAR
T x xx xxx ST
ART oh
yes
STOP
S
T
A
R
T
S
T
O
P

S
T
A
R
T

yes yes yes S
T
O
P
STARTSTOP
START yes STOP
START yes1 STOP START yes2 STOP xxx START yes3 STOP
STOP xxx STOP xxx START xxx START yes4
yes5 STOP  xxx STOP xxx STOP xxx