Hi, what you are looking for is a coercion. (I'd also recommend using Type::Tiny for type constraints).

This can be done in Moose:

use strict;
use warnings;
use feature 'say';

package MyClass {
    use Moose;
    use Types::Standard 'Str';

    has foo => (
        is     => 'rw',
        isa    => Str->where(sub {/^bar\./})->plus_coercions(Str, sub 
+{'bar.' . $_}),
        coerce => 1,
    );
}

my $obj = MyClass->new( foo => 'baz' );

say $obj->foo; # 'bar.baz

$obj->foo('qux');

say $obj->foo; # 'bar.qux'

__END__
[download]

... but IMHO it's much simpler (as are most things) in Moo:

use strict;
use warnings;
use feature 'say';

package MyClass {
    use Moo;
    use Types::Standard 'Str';

    has foo => (
        is     => 'rw',
        isa    => Str,
        coerce => sub {'bar.' . shift},
    );
}

my $obj = MyClass->new( foo => 'baz' );

say $obj->foo; # 'bar.baz'

$obj->foo('qux');

say $obj->foo; # 'bar.qux'

__END__
[download]

Hope this helps!