Right off the bat, this is what I thought of:

sub balance_by {
    my ($self,$entry) = @_;

    my %valid_params = map {$_ => 1} qw/ foo bar /;

    $self->{'balance_by'} = (defined $entry and exists $valid_params{$
+entry})
        ? $entry 
        : 'foo';
    return $self->{'balance_by'};
}
[download]

The only real change is the removal of the unless logic. What that does is set the "balance_by" if you have a valid $entry>, else it sets it to the default "foo". However, it seems to me that you wanted a one-time setting of balance_by that could not be changed. Is that correct? In your code "foo" will only be used as a default on the first pass.

So, if I understood what you wanted, my code will always set balance_by to $entry if you have a valid $entry. If it's not valid, it sets it to "foo".

Cheers,
Ovid

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