You need to test whether the ref value of $data->{foo} is an arrayref: if so, it's a list of messages; otherwise it's a single message. And that's pretty easy:

    if (ref $data->{foo} eq "ARRAY") {
        # list
    }
    else {
        # single message
    }
[download]

What I sometimes like to do in such situations--where I can have either an arrayref or a single value--is coerce the single value into an array ref. Then I can use the same code to deal with both situations:

    unless (ref $data->{foo} eq "ARRAY") {
        $data->{foo} = [ $data->{foo} ];
    }
[download]