$ cat pm_1198523_2.pl
#!env perl
use strict;
use warnings;

my ($R, $C, $sz); # Row, Column and element size
$sz = 5;

for my $r ([0,0], [1,0], [1,1], [2,0], [2,1], [2,2], [12,5]) {
    ($R,$C) = @$r;
    my $seek_addr = seek_addr($R, $C, $sz);
    print "[$R,$C] => $seek_addr\n";
}

sub seek_addr {
    my ($R, $C, $sz) = @_;
    ($R,$C) = ($C,$R) if $R < $C;

    my $slot = $R*($R+1)/2 + $C;
    return $sz * $slot;
}

$ perl pm_1198523_2.pl
[0,0] => 0
[1,0] => 5
[1,1] => 10
[2,0] => 15
[2,1] => 20
[2,2] => 25
[12,5] => 415