fork the new process on the perl side, instead of on the shell:

sub system_background {
  my $pid = fork;
  if(defined($pid) and $pid==0) {
     exec(@_); exit(1)
  }
  $pid
}

for (...) {
  my $child = system_background("perl test1.pl");
  system "perl test2.pl");
  ...
  my $result = waitpid($child, 0);
}
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