Maybe a totally different solution to your problem is to pipe the data. Let the main script generate the test input and then write it to 3 different pipes (or whatever the number of your testclients is). The test client would simply listen on StdIn and process any incoming data packet.

This would neatly sidestep the whole issue of file locking and concurrence. However I really can't tell if this is a suitable solution for your problem even after reading your provided description.

simplified code

open my $fh1, '-|', 'bin/testclient1';
open my $fh2, '-|', 'bin/testclient2';
open my $fh3, '-|', 'bin/testclient3';

my $time1 = 0;
my $time2 = 0;
my $time3 = 0;

while (<condition>) {
  my $testdata = make_foo();

  my $time_a = time;
  print {$fh1} $testdata;
  my $time_b = time;
  print {$fh2} $testdata;
  my $time_c = time;
  print {$fh3} $testdata;
  my $time_d = time;

  $time1 += $time_b - $time_a;
  $time2 += $time_c - $time_b;
  $time3 += $time_d - $time_c;
}

print "method 1: $time1 seconds\n";
print "method 2: $time2 seconds\n";
print "method 3: $time3 seconds\n";

exit;
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P.S.: I always get '-|' and '|-' wrong. You definitely need to check the syntax.