An (Linux) approach without threads (not tested):

LOOP1:while($run_count<max) {

my $pid = `pidof sh1.sh`;

unless ($pid eq ''){
    print "Killing Process sh1.sh PID=$pid\n";
    kill $pid;
    sleep 1;
    next;
}

else {
 $shell_out = system("./sh1.sh");
 $run_count++;
}
}
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