You are right, that previous post has many gem answers. I am just not absolutely sure that they can work in ALL situations where modules load other modules etc. My conclusion was

you must override/alias your sleep() BEFORE use statements for any modules which will use it

The most bullet-proof way I guessed it was to override at compile time and then require all modules which happens at runtime, BEFORE is ensured 100%. However, that reveals side-effects. So I have to write tests for my specific case to ensure that BEFORE ... Thanks (and for your replies in the previous post).

For the record that works and should work as long as it is use'd before any modules whose their sleep needs to be overriden (from LanX's answer at Copy a builtin sub to a different name and then override):

package Override::Sleep;
# based on LanX's answer at https://perlmonks.org/?node_id=1215668
our $total_sleep_time = 0;
our $DEBUG = 0;
BEGIN {
    my $oldsleep = \&CORE::sleep;
    *CORE::GLOBAL::sleep = sub(;$) {
        #$Override::Sleep::total_sleep_time += CORE::sleep($_[0]);
        $Override::Sleep::total_sleep_time += $oldsleep->($_[0]);
        if( $Override::Sleep::DEBUG ){
            my $parent = ( caller(1) )[3] || "N/A";
            print 'CORE::GLOBAL::sleep('.$_[0].") (called by $parent):
+ total sleep time is now ".$Override::Sleep::total_sleep_time." secon
+ds.\n";
        }
       };
}
1;
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