The best idea i've come up with, right now, is using a similar technique to the one used by Damian Conway in the Perl6::Slurp tests with Test::More, and extend it to Test::Class. Here is what he does, to avoid duplicating the test name (Test::Class does this automatically, with the subroutine name) :

my $desc;
sub TEST { $desc = $_[0] };

TEST "can't slurp in void context";
# your test here
ok 1, $desc;
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What i did to extend it to groups of tests in Test::Class, is to use Tests() groups, and having the TEST subroutine declare a test description for each test, and append the subroutine name at the beginning. Like this :

use strict;
use warnings;   #comment before releasing

use Perl6::Say;
use Time::TimeTick;

# TEST :
# my function for displaying information about each test,
# in a Test::Class Test subroutine with several tests
my $desc;
sub TEST { 
    my $test_name   = $_[0]; 
    
    my $caller_func = (caller(1))[3]; # the subroutine full name
    my $test_group  
        = (split('::', $caller_func))[-1]; # only the subroutine name
    $test_group =~ s/_/ /g; # replace underscores by spaces

    $desc = "$test_group  $test_name"; 
};

use base qw(Test::Class);  
use Test::More 0.98;

__PACKAGE__->runtests unless caller;

sub A_Fruit__On_a_branch : Tests(3) {

    TEST "Grows";
    ok 1, $desc;
    
    TEST "Can rot";
    ok 1, $desc;

    TEST "Can fall";
    ok 1, $desc;

}; 

sub A_Fruit__On_the_floor__Rots : Test {
    ok 1;
}

done_testing();
1;
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The output is

1..4
ok 1 - A Fruit  On a branch  Grows
ok 2 - A Fruit  On a branch  Can rot
ok 3 - A Fruit  On a branch  Can fall
ok 4 - A Fruit  On the floor  Rots
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Conclusion :
This does part of what i want : my (one level) nested structure enables me to avoid duplication in test names.
And it doesn't do some of the things that i want (and that Test::Spec does - just check the synopsis) :
- more than one level nested structure.
- different "before" setup functions in different test groups.
- nested "before" setup functions : "The setup work done in each before block cascades from one level to the next"

PS : i just realized, i have no idea to which extent this would work with the Test::Class inheritance process. I guess i'll have to try.

And same thing if i implemented a bit of Test::Spec into Test::Class (i thought of a possible not-too-hard way to implement "describe" and "before each"), i'm not sure that it would work with inheritance. You can inherit a subroutine, but how do you inherit it's context ("describe" and "before each" nested structure) ?