See the Parallel::ForkManager demonstration, here. Something similar can be done with MCE::Child. Subsequently, MCE. Notice the PIDs in the 2nd demonstration where the workers persist.

MCE::Child

use strict;
use warnings;
use MCE::Child qw( );
use Time::HiRes qw( time );
 
my @testLists = qw( sun moon wind air );
my $opts = 'foo';
my $status = 0;

sub regressions {
   my ($opts, $list) = @_;
   print "$$: $list\n";
   sleep 1; # simulate processing
   return 1;
}

my $start = time();

MCE::Child->init(
   max_workers => 2,
   on_finish => sub {
      my ( $pid, $exit_code, $ident, $exit_signal, $core_dump, $data )
+ = @_;
      if ( $exit_code || $exit_signal || $core_dump ) {
         # Handle error.
      }
      $status += $data->{ status };
   }
);
 
for my $list ( @testLists ) {
   MCE::Child->create(sub {
      my $status = regressions( $opts, $list );
      return { status => 1 };
   });
}

MCE::Child->wait_all();
print "status: $status\n";
printf "duration: %0.3f seconds\n", time() - $start;

__END__
27481: sun
27482: moon
27483: wind
27484: air
status: 4
duration: 2.013 seconds
[download]

MCE

use strict;
use warnings;
use MCE::Loop qw( );
use Time::HiRes qw( time );
 
my @testLists = qw( sun moon wind air );
my $opts = 'foo';
my $status = 0;

sub regressions {
   my ($opts, $list) = @_;
   print "$$: $list\n";
   sleep 1; # simulate processing
   return 1;
}

my $start = time();

MCE->new(
   max_workers => 2,
   chunk_size  => 1,
   input_data  => \@testLists,
   gather      => sub { $status += $_[0] },
   user_func   => sub {
      my $list   = $_;
      my $status = regressions( $opts, $list );
      MCE->gather( $status );
   },
)->run();
 
print "status: $status\n";
printf "duration: %0.3f seconds\n", time() - $start;

__END__
27562: sun
27563: moon
27562: wind
27563: air
status: 4
duration: 2.005 seconds
[download]

Indeed, I didn't mean to exclude other modules. I meant the wheel didn't need to be reinvented.

The "$pm->run_on_finish" method takes 2 additional arguments; signal_code and core_dump. I tried the following variation, based on ikegami's solution.

Parallel::ForkManager

use strict;
use warnings;
use Parallel::ForkManager qw( );
use Time::HiRes qw( time );

my @testLists = qw( sun moon wind air );
my $opts = 'foo';
my $status = 0;

sub regressions {
   my ($opts, $list) = @_;
   print "$$: $list\n";
   sleep 1; # simulate processing
   return 1;
}

my $start = time();
my $pm = Parallel::ForkManager->new( 2 );

# P::FM by default, only waits for its own child processes,
# which is what we want.
# Refer to 'BLOCKING CALLS' in the module documentation.
# Let's decrease the sleep period time (default 1.0).
$pm->set_waitpid_blocking_sleep(0.25);

$pm->run_on_finish(sub {
   my ( $pid, $exit_code, $ident, $exit_signal, $core_dump, $data ) = 
+@_;
   if ( $exit_code || $exit_signal || $core_dump ) {
      # Handle error.
   }
   $status += $data->{ status };
});

for my $list ( @testLists ) {
   $pm->start and next;
   my $status = regressions( $opts, $list );
   $pm->finish( 0, { status => $status } );
}

$pm->wait_all_children();
print "status: $status\n";
printf "duration: %0.3f seconds\n", time() - $start;

__END__
27470: sun
27471: moon
27472: wind
27473: air
status: 4
duration: 2.257 seconds
[download]

Thanks, fixed.

One thing that is not clear to me, how it will handle the running of regressions in case number of lists in @teslLists is odd (i.e 3 or 5 or 7). because at last, only one list will be left. so will below line will run only last list even though we have passed 2 to run in parallel.

my $pm = Parallel::ForkManager->new( 2 );
[download]

P::FM forks a child for each "task". If you provide a limit (e.g. the 2), it waits for children to finish before forking if forking would bring the number of children over the limit.

use v5.14;
use warnings;

use Parallel::ForkManager qw( );
use Time::HiRes           qw( sleep );

my $pm = Parallel::ForkManager->new ( 2 );

for ( 1 .. 9 ) {
   $pm->start and next;
   say "[$_] start";
   sleep( $_ == 1 ? 4 : 0.3 );
   say "[$_] done";
   $pm->finish;
}

$pm->wait_all_children();
[download]

Annotated output:

                   1   2   3   4   5   6   7   8   9
           -----  --- --- --- --- --- --- --- --- ---
[1] start  1      XXX
[2] start  1 & 2  XXX XXX
[2] done   1      XXX 
[3] start  1 & 3  XXX     XXX
[3] done   1      XXX 
[4] start  1 & 4  XXX         XXX
[4] done   1      XXX 
[5] start  1 & 5  XXX             XXX
[5] done   1      XXX 
[1] done
[6] start  6                          XXX
[7] start  6 & 7                      XXX XXX
[6] done   7                              XXX
[7] done
[8] start  8                                  XXX
[9] start  8 & 9                              XXX XXX
[8] done   9                                      XXX
[9] done
[download]

Thank you, let me try!