Something like

my $abort = 0;
while (<$fh>) {
    chomp;
    if ($_ eq 'START' .. $_ eq 'END') {
        if ($abort) {
            $abort = 0 if substr($_, -2) eq 'E0';
            next;
        }
        ...
        if (!/cool data/gi) {
            $abort = 1;
            next;
        }
        ...
    }
}
[download]

Of course, it would be much simpler, safer, robust and clearer if you used an XML/HTML parser since you appear to be trying to parse XML/HTML.