Much easier as "forked open"

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1148787

use strict;
use warnings;

my $total = 0.0;
my @waitlist = qw(0 200 400 800 1000);
my @fhs;

foreach my $item (@waitlist)
  {   
  if( open my $fh, '-|' )
    {   
    push @fhs, $fh;
    }   
  else
    {   
    my $h = 1.0 / 1200.0;
    my $sum = 0.0;
    for my $i ($item .. $item + 200 )
      {         
      my $x = $h * ($i - 0.5);
      $sum += 4.0 / (1.0 + $x*$x);
      }   
    my $ret=$sum * $h; 
    print $ret;
    #warn "$item exiting\n";
    exit; 
    }   
  }

for my $fh ( @fhs )
  {
  $total += (<$fh> + 0.0);
  print "$total\n";
  close $fh;
  }
   
print " $total\n";
print "All done!\n";
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