use Foo;
use Foo ':modern';

##</code><code>##

package Foo;

use strict;
use warnings;
use feature 'say';
use parent 'Exporter';

my $MODE = 0;

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

sub import {
    my ( $self, $flag ) = @_;
    
    if ( defined $flag and $flag eq ':modern' ) {
        $MODE = 1;
        Foo-> export_to_level( 1, 'Foo', keys %const );

        no warnings 'redefine'; 
        *Foo::m2 = \&Foo::Modern::m2;
        # etc.
        # longer list
    }
}

sub new { my $class = shift; bless {}, $class }
sub m1 { say $MODE ? 'modern 1' : 'classic 1' }
sub m2 { say 'classic 2' }

package Foo::Modern {

sub m2 { say 'modern 2' }

};

1;

##</code><code>##

use strict;
use warnings;
use feature 'say';

use lib '.';
use Foo ':modern';

my $obj = Foo->new;
$obj->m1;
$obj->m2;
say ABC;