I think you could just do something like this:

    sub pack {
        my $self = shift;
        return join ':',
               map $self->{$_},
               keys %{ $self->{_permitted} };
    }
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$self->{_permitted} is a reference to %fields. So we take the keys in %fields (the names of your data members), and for each of those we get the values of those fields held by your object; then we join those values into a colon-separated string. Is that what you wanted?