$condition{$key} = \@{$query->param($key)} || $condition{$key} = $query->param($key);

##</code><code>##

$condition{$key} = [$query->param($key)];

##</code><code>##

if (@{$condition{$key}} == 1) {
   # one value
}
else {
   # ... zero or more than one value
}