bless do { my %self = (%{Colour->new($colour)}, %{Age->new($age)}); \%
+self }, $class;
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So, you break encapsulation, and will depend on how things are implemented. If you're willing to break encapsulation, why even bother with OO?

The other solution is to go the inside-out object route. If your class doesn't care about the contents of the reference, then all is fine and dandy.

Inside-out objects aren't a silver bullet, and won't save the naive programmer.

package Colour {
    use Scalar::Util 'refaddr';
    my %colour;
    sub new {
        my $o = bless \do{my $var}, $_[0];
        $colour{refaddr $o} = $_[1];
        $o;
    }
    sub colour {$colour{refaddr $_[0]}}
};
package Age {
    use Scalar::Util 'refaddr';
    my %age;
    sub new {
        my $o = bless \do{my $var}, $_[0];
        $age{refaddr $o} = $_[1];
        $o;
    }
    sub age {$age{refaddr $_[0]}}
};
package Colour_and_Age {
    our @ISA = qw[Age Colour];
    use Scalar::Util 'refaddr';
    sub new {
        my ($class, $colour, $age) = @_;
        ... Now what? ...
    }
}
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Packages Colour and Age are written as inside-out objects, but because the same method constructs and initializes the object, the author of Colour_and_Age is screwed. He doesn't even have the luxury of breaking encapsulation.

Now, if one separates object construction and object initialization, for instance:

use Scalar::Util 'refaddr';
package Colour {
    use Scalar::Util 'refaddr';
    my %colour;
    sub new {bless \do{my $var}, $_[0]}
    sub init {$colour{refaddr $_[0]} = $_[1]; $_[0]}
    sub colour {$colour{refaddr $_[0]}}
};
package Age {
    use Scalar::Util 'refaddr';
    my %age;
    sub new {bless \do{my $var}, $_[0]}
    sub init {$age{refaddr $_[0]} = $_[1]; $_[0]}
    sub age {$age{refaddr $_[0]}}
};
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then one can use multiple inheritance without any problem:

package Colour_and_Age {
    use Scalar::Util 'refaddr';
    our @ISA = qw[Age Colour];
    sub new {bless \do{my $var}, $_[0]}
    sub init (
        my ($self, $colour, $age) = @_;
        $self->Colour::init($colour);
        $self->Age::init($age);
        $self;
    }
};

my $o = Colour_and_Age::->new->init(red => 42);
[download]

It only takes a few more keystrokes to create your classes that way, and you aren't robbing any users from using your classes with multiple inheritance. Also note you don't need inside-out objects to separate constructions and initialization.