wps.io post - mxas && Mooish constraints http://wps.io/2015/04/mxattributeshortcuts-now-with-moo-style-type-constraints/

isa-moo.pl

# easiest is via AttributeShortcuts
use MooseX::AttributeShortcuts 0.028;
 
has foo => (
    is  => 'rw',
    # $_ == $_[0] == the value to be validated
    isa => sub { die unless $_[0] == 5 },
);