use warnings;
use strict;

my $flag = 0;
my @lines;
while (<DATA>) {
    if (/KEY/) {
        @lines = ();
        $flag = 1;
    }
    if ($flag) {
        push @lines, $_;
        if (/PATTERN/) {
            print @lines;
            $flag = 0;
        }
    }
}

__DATA__
KEY
blah
blahblah
KEY
blah ah
other random stuff
KEY
blah ha
other random stuff
PATTERN
asdf
KEY
fdas
PATTERN

##</code><code>##

KEY
blah ha
other random stuff
PATTERN
KEY
fdas
PATTERN