# ./lib/Foo/Classic.pm
# --------------------
package Foo::Classic;
use warnings;
use strict;
use feature qw{ say };
use experimental qw( signatures );

use parent 'Foo::Common';

sub style($self) { 'classic' }
sub m2($self)    { say 'classic 2' }

__PACKAGE__

# ./lib/Foo/Constants.pm
# ----------------------
package Foo::Constants;
use warnings;
use strict;

use Exporter qw{ import };

my %const;
BEGIN {
    %const = (
        ABC => 'abc',
        XYZ => 'xyz',
    );
}
use constant \%const;
our @EXPORT_OK = keys %const;

__PACKAGE__

# ./lib/Foo/Modern.pm
# -------------------
package Foo::Modern;
use warnings;
use strict;
use feature qw{ say };
use experimental qw( signatures );

use parent 'Foo::Common';

sub style($self) { 'modern' }
sub m2($self)    { say 'modern 2' }

__PACKAGE__

# ./lib/Foo/Builder.pm
# --------------------
package Foo::Builder;
use warnings;
use strict;
use experimental qw( signatures );

use Foo::Classic;
use Foo::Modern;

my %CLASS = (classic => 'Foo::Classic',
             modern  => 'Foo::Modern');
sub new($class) {
    return bless {}, $class
}

sub build($self, $style, @args) {
    my $class = $CLASS{$style};
    die "Unknown style $style" unless $class;
    return $class->new(@args)
}
__PACKAGE__

# ./lib/Foo/Common.pm
# -------------------
package Foo::Common;
use warnings;
use strict;
use feature qw{ say };
use experimental qw( signatures );

sub new($class)  { bless {}, $class }
sub m1($self)    { say $self->style, ' 1' }
sub style($self) { die 'Not implemented' }

__PACKAGE__

# ./script.pl
# -----------
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use lib 'lib';
use Foo::Builder;
use Foo::Constants qw( ABC XYZ );

my $builder = 'Foo::Builder'->new;

my $m = $builder->build('modern');
$m->m1;
$m->m2;

my $c = $builder->build('classic');
$c->m1;
$c->m2;

say ABC;