$ cat pm_1198523.pl
#!env perl
use strict;
use warnings;

my $rData = [
    [ 1, 3, 5, 7, 9 ],
    [ 0, 2, 4, 6, 8 ],
    [ 0, 0, 3, 6, 9 ],
    [ 0, 0, 0, 8, 4 ],
    [ 0, 0, 0, 0, 3 ]
];

bless $rData, 'foo';

print $rData->fetch(1,4), "\n";
print $rData->fetch(4,1), "\n";

print $rData->fetch(3,1), "\n";
print $rData->fetch(1,3), "\n";

package foo;

sub fetch {
    my ($self, $r, $c) = @_;
    ($r, $c) = ($c, $r) if $c < $r;
    return $self->[$r][$c];
}

$ perl pm_1198523.pl
8
8
6
6