We need it because we have to report to the overlords how much testing we are doing.

Using the plan in the subtests doesn't work:

$ cat test.t
use Test::More;

subtest foo => sub {
    plan_tests => 3;
    ok 1;
    ok 2;
    ok 3;
};

$ prove -lrv test.t
test.t ..
# Subtest: foo
    ok 1
    ok 2
    ok 3
    1..3
ok 1 - foo
# Tests were run but no plan was declared and done_testing() was not s
+een.
Dubious, test returned 254 (wstat 65024, 0xfe00)
All 1 subtests passed
[download]

$ cat test.t

use Test::More;

subtest foo => sub {
    plan_tests => 3;
    ok 1;
    ok 2;
    ok 3;
};

done_testing;

$ prove -lrv test.t
test.t ..
# Subtest: foo
    ok 1
    ok 2
    ok 3
    1..3
ok 1 - foo
1..1
ok
All tests successful.
Files=1, Tests=1, ...
[download]

What to you mean by "doesn't work"? It clearly works in the second snippet. If you want to report the number of single tests, process the output for /\d+\.\.\d+/. Something like this:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use List::Util qw{ sum };

my %test;
while (<>) {
    if (/^( *)([0-9]+)\.\.([0-9]+)$/) {
        my $length = length $1;
        my ($from, $to) = ($2, $3);
        $test{$length} += $to - $from + 1;
        --$test{$length - 4} if $length;
    }
}
say sum(values %test), ' tests run.';
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I only did minimal testing, so test it properly.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]