Please read Markup in the Monastery as you've just made a mess of my browser window.

I'm pretty sure you don't want an if-elsif-else set here, unless you want to keep at most one value from each line. You also probably mean to have a hash of hashes of hashes structure here in place of what you are doing (perllol). The test you are performing actually checks if an index exists yet, not if the value is the array. The code you want is likely closer to:

while (<FICHEROCFG>) {
    my $datos_cfg = $_;
    my ($var1,$var2,$var3,$var4) = split (",", $datos_cfg);
    $db{$var1}{'Entidad'}{$var2} = 1;
    $db{$var1}{'Agrupacion'}{$var3} = 1;
    $db{$var1}{'Operativa'}{$var4} = 1;
}
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Update: And to answer your updated question, the index of the last element of @array is given by $#array.

If I understand what you're doing, I think, yes, it will do what you want.

exists($list[$num])
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Will tell if there is an item of some kind, possibly undefined, at index $num in list $list.

I think $#{$arrayref} is what you're looking for with the 2nd question. EG $#{$hash_of_lists{$foo}}.

The issue is that push @list, $num doesn't set element $list[$num], so the tests don't correlate. If instead the OP used $list[$num] = 1; or equivalent, then [she|he] would get a false positive with exists if [she|he] set $list[5] and then checked $list[3] - defined would give correct results, though.

Oy, yes. I see what you mean. The array will grow to accomodate it's highest index, and therefore all lower indexes will come into existence.

I think the OP should use a hash. Clearly, it's a hash-ish behavior he/she wants. The fact that the keys are numbers is making it look like a list problem, but really, it's a hash problem.

Regarding Update 2:

As a second note, so long as your hash keys contain only alphanumerics and underscores, Perl will automatically stringify the result - there's no need for quotes.

thanks for the quotes trick!