Update: Actually, no. You need to remove the three lines mentioning $last:

my @history;
my $expect = 1;
while (<>) {
    chomp;

    my ($num) = /EVENT(\d+)/
        or next;  # Ignore bad input.

    if ($num == 1 || $num != $expect) {
        @history = ();
        $expect = 1;
    }

    if ($num == $expect) {
        push @history, "$_\n";
        if ($expect++ == 4) {
            print(@history);
            @history = ();
            $expect = 1;
        }
    }
}
[download]

The funny thing is that I didn't have those three lines originally since I had forgotten that part of your question when I first wrote up my reply.

Many, many thanks! That did it.

Only because he edited his old post. It used to be a different question.

What algorithms have you tried?

As I understand it, all you want to do is:

• If the line is NOT event #{counter} set counter to undef/zero
• If the line is event #1, set counter to 1
• If the line is event #{counter}, increment counter
• If the counter is now greater than {num events: IE 4}, save the last four lines off to somewhere and reset the counter to undef/zero