Use closures. Check out perltoot for more info on how this code works.

{
  package Foo;

  sub new {
    my $pkg = shift;
    my %data = (
        bar => undef,
    );

    bless sub {
        my $field = shift;
        @_ and $data{$field} = shift;
        $data{$field}
    }, $pkg
  }

  sub bar {
    my $self = shift; # the blessed closure
    $self->( 'bar', @_ )
  }
}
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my $foo = new Foo;

$foo->bar(42);   # set it
print $foo->bar; # get it
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