gistfile1.pl

#!/usr/bin/perl

use feature ':5.10';
use MooseX::Declare;

class Foo {
    method _test { 1 }

    method bar ( $f where { _test(')}') } ) {
        $f
    }
}

say Foo->new->bar('22');

# Blows up with:
# Runaway '{}' in parameter near '{ _test(')}')' at /usr/local/share/perl/5.10.0/MooseX/Method/Signatures/Meta/Method.pm line 160