I've had to do exactly that with Class::MethodMaker. I needed to initialize an object with a LOT of data (and a LOT of method calls) and initializing the object through repeated method calls was S-L-O-W. So, I subverted the regular constructor with my own:

package Datawarehouse::Employee;
use Class::MethodMaker
    new_with_init => 'new'; 
    
sub init {
    # ...Load up a whole lot of data

    # Bless my own object into the class
    Datawarehouse::Employee->new_new(\%args);
}

sub new_new {
    # This constructor is a HELL of alot faster than creating an
    # object by calling accessor methods one by one.
    my $class = shift;
    my $args = shift;
    my $obj = bless $args, $class; # Loading the whole hash as one
}
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If you do this while using the key_attrib parameter in Class::MethodMaker you'll need to call the key_attrib method explicitly so that your hand-made object is tracked like the regular one's are. Looking at the Class::MethodMaker source code is a great way of figuring out exactly what's going on with the autogenerated methods.