In Moose, how do I modify an attribute any time it is set?

If you have an attribute that needs to be modified any time it is set, is there a slick way of doing this short of writing the accessor yourself and mucking around directly with the content of $self, as done in this example?

package Foo;
use Moose;

has 'bar' => (
    isa => 'Str',
    reader => 'get_bar',
);

sub set_bar {
    my ($self, $bar) = @_;
    $self->{bar} = "modified: $bar";
}

I considered trigger, but it seemed to require the same approach.

Is working directly with the hash reference in $self considered bad practice in Moose, or am I worrying about a non-issue?