sub conf {
  shift; # throw away unused class.
  # .. do real work here
}

##</code><code>##

my %re_use = (name => 'Alice');
Ex::Mod->new( override=>'whatever', %re_use );

##</code><code>##

use strict;
use warnings;
use lib '.';
require Ex::Mod;

Ex::mod::conf( user => 'Alice' );
my $a = Ex::mod->new();
# Presumably more constructor use wanting user=>'Alice' goes here..
my $b = Ex::mod->new( user=>'Bob');
printf "User a is: %s\n", $a->whoami();
printf "User b is: %s\n", $b->whoami();

##</code><code>##

package Ex::mod;
use strict;
use warnings;

# package-scoped config.
my %conf = (
	user => undef,
);

# config routine. NB: FP!
# <class>::conf( 'key' => 'value' );
sub conf {
	my %c = ( @_ );
	for (keys %c) {
		$conf{$_} = $c{$_} if exists $conf{$_};
	}
}

# new(), the constructor.
sub new {
	my $class = shift;
	$class = ref($class) || $class;	# subclass boilerplate.
	my %opts = ( @_ );
	my $self = {
		user => $opts{user} // $conf{user} // '',
	};
	return bless $self, $class;
}

# whoami() - method to say who this object's user is:
sub whoami {
	my $self = shift;
	return $self->{user} // undef;
}