The code which you show is a common idiom which hides a lot of details. Here is an expanded version which should be easier to follow. 
$type Damnitaddie.pl
use strict;
use warnings;
use Data::Dumper;

my @rows = ( [qw(a b c  d)], [qw(e f c  h)], [qw(i j k  l)], );

my %seen;
@rows = grep {
    my $x = $_->[2];
    if ( exists $seen{$x} ) {
        $seen{$x}++;
        0;
    }
    else {
        $seen{$x} = 0;
        $seen{$x}++;
        1;
    }
} @rows;

#@rows = do { my %seen; grep { !$seen{$_->[2]}++ } @rows };

print Dumper( \@rows );

$perl Damnitaddie.pl
$VAR1 = [
          [
            'a',
            'b',
            'c',
            'd'
          ],
          [
            'i',
            'j',
            'k',
            'l'
          ]
        ];

[download]
Bill

In reply to Re: Removing duplicates in multi-dimensional arrays by BillKSmith
in thread Removing duplicates in multi-dimensional arrays by DamnitAddie

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.