It's a little difficult to parse :), and as ever,it'd help if you posted some more code, but maybe you're simply looking for something like...

if ($dbm{$accountAD} eq $accountID){
    do_something(); # got an existing one;
}
elsif (defined $dbm{$accountAD}) {
    do_something_else(); # a mismatch?
}
else {
   $dbm{$accountAD} = $accountID; #set a new one
}
[download]

Hope this helps.