This is an ideal case for extract_tagged method of Text::Balanced. From the perldoc, I think you should say something like:

my $remainder = $original_string;
my @answers;
while ( defined $remainder ) {
    my $extracted;
    ( $extracted, $remainder )
        = extract_tagged(
            $remainder,
            'START',
            'END',
            undef,
            { bad => ['START'] },
          );
    push @answers, $extracted;
}
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Phil