use strict;
use warnings;

use Test::More tests => 1;

my $lines = [
    'c1r1,c2r1,c3r1,c4r1,c5r1',
    'c1r2,c2r2,c3r2,c4r2,c5r2',
    'c1r3,c2r3,c3r3,c4r3,c5r3'
];
my @store;

foreach my $line (@$lines) {
    my @val = split (/,/, $line);
    push @store, $val[2];
}

is_deeply (\@store, ['c3r1', 'c3r2', 'c3r3']);
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See How to ask better questions using Test::More and sample data.

You didn't need to escape the comma in the regex. You don't need the brackets in the push statement. A single array element is better written with a $ sigil. That aside, your code (as demonstrated here) works fine.