You can do it all in memory if the list isn't too big. Since it's going to fit in a drop-down box, I presume that's the case. Also, I presume a simple split on the ! character will serve to isolate the fields appropriately (in other words, there are no embedded !s).

Then the trick is to use a hash to remember each distinct occurence. Afterwards you can look at the keys in the hash to retrieve all the unique values. The combination would be something like this (note, this is untested):

my %unique;
while (<IN>) {  # or whatever the file handle is
    my @fields = split /!/;
    $unique{$fields[-1]}++;   # last field in the row
}
my @drop_down = keys %unique; # Et voila
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HTH