Here is the bare bones way to do it. You didn't say exactly what your problem was in the code.

#!/usr/bin/perl
use warnings;
use strict;


my %pids;

my @to_be_processed = (1..20);

my $cmd = 'echo';

foreach my $p (@to_be_processed){ 
   $pids{$p}{'pid'} = open($pids{$p}{'fh'}, "$cmd $p 2>&1 |");
}

foreach my $key(keys %pids){ 
  my $fh = $pids{$key}{'fh'};
  while (<$fh>){
    print $_;
    #stuff data here into a return hash
   }
}



foreach my $key(keys %pids){
   waitpid($pids{$key}, 1);
}

print "done\n";
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