package test;

use Moose;
use Moose::Util::TypeConstraints;

subtype 'mytype'
    => as 'HashRef[Str]'
;

coerce 'mytype'
    => from 'Str'
    => via {
        my %h = map { split('=', $_) } split(',', $_);
        return \%h;
    }
;

has 'mytype' => (
    traits => ['Hash'],
    is => 'rw',
    isa => 'mytype',
    coerce => 1,
    # presumably I need an explicit accessor
    # to prevent coercion when I pass
    # a key in to access the value?
    handles   => {
        get_option     => 'get',
    },
);

package main;
use Data::Dumper;

my $t = test->new;

# coerce into a hash
$t->mytype('foo=bar,foo2=bar2');

warn Dumper $t->mytype;

# how do i now access the hash by key?
# this tries to coerce all over again!
warn $t->mytype('foo');

# and this doesn't work at all
warn $t->mytype_get('foo');
[download]

Your module works fine:

# ... all same above ...
package main;
use Data::Dumper;

my $t = test->new;

$t->mytype('foo=bar,foo2=bar2');

warn Dumper $t->mytype;

# Don't call $t->mtype('foo')

# Use correct method name!!
warn $t->get_option('foo');
[download]

Good Day,
    Dean

Ah, doh! Thanks - much appreciated!