on-the-fly subtyping

gistfile1.pl

use MooseX::AttributeShortcuts;
use MooseX::CoercePerAttribute;
use MooseX::AlwaysCoerce;

# create an attribute where value must be a subtype of Str that ends in '.',
# and can be coerced from a non-conformant Str by adding a '.'

has foo => (
    is => 'ro',
    isa => 'Str',
    constraint => sub { /\.$/ },
    coerce => {
        Str => sub { "$_." },
    },
    builder => sub { 'some string' },
);