A solution using Proc::Queue:

One process is created to perform the data load while several (4) processes handle the data conversion. The conversors tell to the loader when they finish through a pipe.

Proc::Queue is used to ensure that only 4 conversors + the loader run at the same time

# untested:
use Proc::Queue size => 4, qw(run_back);

my $lpid = open my $loader, '|-';
defined $lpid or die "unable to fork: $!";

if ($lpid) {
  select $loader; $| = 1; select STDOUT;
  for (@ARGV) {
    run_back {
      if (system("convert.sh $payload") == 0) {
        print $loader "$payload\n";
      }
      else {
        warn "Couldn't launch: $!/$?";
      }
    } or warn "unable to fork: $!/$?";
  }
}
else {
  while (<>) {
    system "load_db.sh $_") == 0
      or warn "Couldn't launch: $!/$?";
  }
}

1 while wait != -1;
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