Having said that I still think that lookahead and lookbehind are the best bet.

Just join them like this instead:

$total_raw = "(" .   # WRONG
    join ("|",
        map (
            "(?<!\Q$pre[$_]\E)ALPHA(?!\Q$suf[$_]\E)",
            (0..$#pre)
        )
    )
    . ")";
$total_rx = qr/$total_raw/;
[download]

(untested) Or something like that.

Update: Dooh. That doesn't work. Need an "and" match not an "or" match. Hmmmm... back to the drawing board.

Update: Okay, I've got it...

Can you nest lookaheads and lookbehinds? If so this should work:

$total_raw = "(?!(" .  # CORRECT, MAYBE
    join ("|",
        map (
            "(?<=\Q$pre[$_]\E)ALPHA(?=\Q$suf[$_]\E)",
            (0..$#pre)
        )
    )
    . "))ALPHA";
$total_rx = qr/$total_raw/;
[download]

This has a zero-width assertion followed by ALPHA.

The zero-width assertion is a lookahead exclude (ie looking ahead, that this is not true)

What it is looking ahead to see is one of multiple patterns.

Each pattern contains an ALPHA with a lookahead and behind for an excluded suffix and prefix pair. If one of these matches, we have an excluded alpha - so we exclude this match (the original zero-width assertion).

So it says: match an ALPHA that does not have an excluded prefix/suffix.

-Andrew.