I was gonna suggest using the range op, but it doesn't really help

my @lines;
while (<DATA>) {
    my $is_key = /KEY/;
    @lines = () if $is_key;
    if (my $in_range = $is_key .. /PATTERN/) {
        push @lines, $_;
        print @lines if $in_range =~ /E0\z/;
    }
}
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If one continues to simplify the above, one gets the parent's code.