Here's another way to do it:

#!/usr/bin/env perl -l

use strict;
use warnings;

my @values;

while (<DATA>) {
    chomp;
    push @values, $_;

    if (@values == 3) {
        process_values(@values);
        @values = ();
    } 
}

process_values(@values) if @values;

sub process_values {
    my ($i, $j, $k) = @_;
    print '$i = ', defined $i ? $i : '<undefined>';
    print '$j = ', defined $j ? $j : '<undefined>';
    print '$k = ', defined $k ? $k : '<undefined>';
}

__DATA__
a
b
c
d
e
f
g
h
[download]

Output:

$i = a
$j = b
$k = c
$i = d
$j = e
$k = f
$i = g
$j = h
$k = <undefined>
[download]