You can see in the MooseX Declare documentation that it implicitly uses namespace::autoclean. This automatic cleaning of imported subs ends up removing the special subs installed by overload as well. There is a bug filed on namespace::autoclean to fix this issue. Another workaround aside from what ikegami posted would be to use the 'is dirty' modifier to avoid the automatic cleaning.

Test: 
use MooseX::Declare;
class InDeclare {
    use overload '""' => \&to_string2;

    has val => is => 'rw';
    sub to_string2 { sprintf "(%s)", $_[0]->val }
};

class InDeclareDirty is dirty {
    use overload '""' => \&to_string2;

    has val => is => 'rw';
    sub to_string2 { sprintf "(%s)", $_[0]->val }
};


package main;
use strict;
use warnings;
use Test::More;

my $d = InDeclare->new(val=>666);
is $d,   "(666)", "mxd is ok";    # PASS: "(666)"
is "$d", "(666)", "mxd is ok (Q)"; # FAIL: "InDeclare=HASH(0x2166ab0)"

my $dd = InDeclareDirty->new(val=>6666);
is $dd,   "(6666)", "mxdd is ok";
is "$dd", "(6666)", "mxdd is ok (Q)";

done_testing;

[download]
Results: 
ok 1 - mxd is ok
not ok 2 - mxd is ok (Q)
#   Failed test 'mxd is ok (Q)'
#   at test.pl line 24.
#          got: 'InDeclare=HASH(0x100e42bd0)'
#     expected: '(666)'
ok 3 - mxdd is ok
ok 4 - mxdd is ok (Q)
1..4
# Looks like you failed 1 test of 4.

[download]

In reply to Re: MooseX::Declare and overload by Haarg
in thread MooseX::Declare and overload by FunkyMonk

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