Structured programming^† (as recommended by Dijkstra in his "Go To Considered Harmful") would want you to write it as:

foreach my $a (@b) {
    if (!...) {
        do_something();
        if (!...) {
            do_something_else();
            if (!...) {
                do_something_again();
            }
        }
    }
    perform_cleanup();
}
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^†The essence being that each block has exactly one entry, and exactly one exit point.