I think you probably want parentheses instead of square brackets around \\\&, like this:

$x = 'o\&a.sysadm';

if ($x =~ m/^(\w+(?:\\\&)?-?\w*)\.((?:\w+\\?(?:\\\&)?-?\w*\.?)+)/i){
    $client = $1;
    $project = $2;
}
[download]

Update: changed a [...] I had missed into (?:...).

-- Mike

--
just,my${.02}