Hash free !

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1138357

use strict;
use warnings;

$_ = <<END;
 PID  PPID  COMMAND
   0     0  sched
   1     0  /sbin/init
   7     0  vmtasks
 105     1  /usr/lib/saf/sac
7184     1  /usr/bin/java
7222     1  /usr/lib/utmpd
7501  6223  /usr/sbin/nscd
7507  7184  /bin/sh
7508  7507  /usr/bin/perl
7510  5044  /usr/bin/grep
7512  4333  /usr/bin/egrep
7515  7508  sh
7516  7515  <defunct>
END

for my $defunct ( /^\s*(\d+)\s+\d+\s+<defunct>/gm )
  {
  my $pid = $defunct;
  my $answer = '';

  while( $pid > 0 && /^(\s*$pid\s+(\d+).*\n)/m )
    {
    $answer = $1 . $answer;
    $pid = $2;
    }
  print "$answer\n";
  }
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