exec is enough to make me think this is a bad idea. And there are other reasons why this wouldn't really address the "problem".

Instead write Test::AutoPlan that you use like:

perl -MTest::AutoPlan -e0 t/*
[download]

and it would run each of your t/* files and then modify them to record the current plan count, notifying you how the counts had changed.

Instead of writing your t/* files like:

use Test::More ( ... );
...
[download]

you'd write them like:

use Test::AutoPlan qw( Test::More ... );
...
[download]

and perl -MTest::AutoPlan t/* would change that to

use Test::AutoPlan 23 qw( Test::More ... );
...
[download]

and you could even make it so "make plan" updates the test count plans.

The Test::AutoPlan code goes something like this:

my $plan;

sub VERSION {
    $plan = $_[1];
}

sub import {
    my $self = shift(@_);
    goto &UpdateTests   if  ! @_;
    my $module = shift(@_);
    my @plan = "no_plan";
    if(  defined($plan)  ) {
        @plan = ( tests => $plan );
    } elsif(  "Test::Simple" eq $module  ) {
        @plan = ( tests => 0 );
    }
    unshift @_, $module, @plan;
    my $import = $module . "::import";
    undef $plan;
    goto &$import;
}

sub UpdateTests {
    for my $test ( @ARGV ) {
        my $plan = RunTestsAndCount( $test );
        @ARGV = $test;
        $^I = ".old";
        while( <> ) {
            s{
                ^(
                  [\w\s]*
                  (?<![\w:])
                  Test::AutoPlan
                )
                (\s+\d+)?
            }{
                $1 . " " . $plan
            }ex;
            print;
        }
    }
}
[download]

The first argument could be Test::Simple instead of Test::More (or whatever other modules you decide to support).