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

##</code><code>##

'STARTblah haother random stuffSTOP'
'STARTSTOP'
'STARTfdasSTOP'
'START yesSTOP'
'START ohyesSTOP'
'STARTSTOP'
'STARTyes yes yes STOP'
'STARTSTOP'
'START yes STOP'
'START yes1 STOP'
'START yes2 STOP'
'START yes3 STOP'
'START yes4yes5 STOP'