#!/bin/env perl
use strict;
use warnings;
my $start= qr/^KEY$/;
my $end  = qr/^PATTERN$/;

my $match;
while (<DATA>) {
        if (my $hit= /$start/ .. /$end/) {
                $match= '' if /$start/;
                $match.= $_;
                print $match if $hit ne 0+$hit;
        }
}
__DATA__
KEY
blah
blahblah
KEY
blah ah
other random stuff
KEY
blah ha
other random stuff
PATTERN
asdf
KEY
fdas
PATTERN