You should put your MooseX-Types library into a separate file. The method signatures are parsed at compile time, and if they are defined in the same package, they are not declared yet.

Yes, as some anonymous monk said above, that's why I put types declaration into BEGIN block (inlined for brevity). In real code these declarations are in a separate file.

That is why BEGIN{} blocks were invented :)

BEGIN {
    use MooseX::Declare;

    class Foo {
        use feature qw(say);

        use Data::Dumper;

        use MooseX::MultiMethods;
        use MooseX::Types -declare => [qw(Bar MyStr MyNum)];
        use MooseX::Types::Moose qw(Str Num);
        use MooseX::Types::Structured qw(Dict);

        BEGIN {
            subtype MyStr,
                as Str;
            subtype MyNum,
                as Num;

            subtype Bar,
                as Dict[
                    string => MyStr,
                    number => MyNum,
                ],
                message { 'Invalid Foo struct' };
        };

        multi method baz(Bar $barrr) {
            say'Bar';
            say Dumper $barrr
        }

        multi method baz(Str $string) {
            say 'Str';
            say Dumper $string;
        }
    }

}
no Moose;
no MooseX::Declare;

1;

package main;

my $foo = Foo->new;
$foo->baz( { string => 'hello', number => 1.00, } );
$foo->baz( 'hello world' )
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