I'd use several different data structures, a hash of arrays, an array, and an array of arrays.

use strict;
my %byPid = (
       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>' ],
);
my @defunct = ( 7516 );
for my $d ( @defunct ) {
    my @layers;
    my $pid = $d;
    while( 1 ) {
        my( $parent, $cmd ) = @{ $byPid{$pid} }
            or  last;
        unshift @layers, [ $pid, $parent, $cmd ];
        last
            if  $pid == $parent;
        $pid = $parent;
    }
    printf "%6s %6s %s\n", 'PID', 'PPID', 'Cmd';
    for my $layer ( @layers ) {
        printf "%6d %6d %s\n", @$layer;
    }
    print "\n";
}
__END__
   PID   PPID Cmd
     0      0 sched
     1      0 /sbin/init
  7184      1 /usr/bin/java
  7507   7184 /bin/sh
  7508   7507 /usr/bin/perl
  7515   7508 sh
  7516   7515 <defunct>
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