Following your advises and adding what I received on #perl irc channel, I both made the eventually failing test not critical (using SKIP ) and I also let the user to provide their custom rand function. Relevant part are:

# ./Games/Dice/Roller.pm

sub new{
    my $class = shift;
    my %opts = @_;
    if ( defined $opts{sub_rand} ){
        croak "sub_rand must be a code reference meant to replace core
+ rand function"
            unless ref $opts{sub_rand} eq 'CODE';
    }
    return bless {
        sub_rand =>  $opts{sub_rand} // sub{ rand($_[0]) },
    }, $class;
}

sub single_die{
    my $self = shift;
    my $sides = shift;
    croak "single_die expect one argument" unless $sides;
    croak "Invalid side [$sides]" unless $sides =~/^(\d+)$/;
    $sides = $1;
    return 1 + int( $self->{sub_rand}($sides) );
}


# ./t/01-single-die.t

my $tenk;
$tenk += $dice->single_die(6) for 1..10000;
my $avg = $tenk/10000;
# $avg = 5; # uncomment this line to provoke the warning
if ( ($avg < 3.4) or ($avg > 3.6) ){
    diag(
            "\n\n\nPROBLEM: you got an average of $avg while was expec
+ted a value > 3.4 and < 3.6\n\n\n".
            "The average was made on 10000d6 rolls.\n".
            "This can happen in old Perl distribution on some platform
+.\n".
            "You can use sub_rand => sub{.. during constructor to prov
+ide an\n".
            "alternative to core rand function (using rand from Math::
+Random::MT for example).\n\n\n\n"
    );
}
else { 
        pass "average randomness ok (3.4 < 10000d6 / 10000 < 3.6)" ;
}


# ./t/07-rand-custom-funcion.t

SKIP: {
    eval { require Math::Random::MT };
    skip "Math::Random::MT not installed", 2 if $@;
    my $gen = Math::Random::MT->new();
    my $mt_dicer =  Games::Dice::Roller->new(
        sub_rand => sub{ 
                my $sides = shift; 
                return $gen->rand( $sides );            
        },
    );
    my ($res, $descr) = $mt_dicer->roll('13d4kh7');
    ok( $res >= 7, "succesfully used rand from Math::Random::MT as ran
+dom number generator");
    ok( $res <= 28, "succesfully used rand from Math::Random::MT as ra
+ndom number generator")
}
[download]

The house of this toy project (for hippo' happines ;) is at gitlab

Thanks to you all

L*