my $name = "multi";  # Renamed from $key to be less confusing

# Sorry, I don't want to show my /etc/password :)
my %user = (
    'fred'     => 101,
    'barney'   => 102,
    'wilma'    => 103,
    'betty'    => 104,
    'pebbles'  => 105,
    'bambam'   => 106,
);

...
<select name="$name" size="15" multiple="multiple">

foreach my $key(sort keys %user) {
    my $val = $user{$key};
    if ($val > 100) {
        print qq{<option value="$key">$key ($val)</option><br />};
    }
}