I assume you want to set the value if a parameter is passed or if the current value is undef, and just return the current value otherwise:

sub balance_by {
  my $self = shift;
  my %valid_params = map {$_ => 1} ('foo',
                    'bar');

  if (@_ or ! defined $self->{'balance_by'}) {
    my $entry = shift;
    $self->{'balance_by'} = 
        (defined $entry and $valid_params{$entry})
            ? $entry 
        : 'foo';
  }
  return $self->{'balance_by'};
}
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I would also define your %valid_params outside of the subroutine. No need to define them each time you call the sub. Also if you want to nano-optimize your code, consider using exists to see if the key exists in valid_params instead of testing its value, like so:

my %valid_params; undef @valid_params{qw(foo bar)};
...
(defined $entry and exists $valid_params{$entry})
...
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Also, unless I'm mistaken, there doesn't seem to be any reason to not default your fields in the 'new' method...(unless you just wanted 'if (@_)...' in that conditional?)