Note: This is the type of transform that map was designed for.

use strict;
use warnings;
my $lines = [
    "c1r1,c2r1,c3r1,c4r1,c5r1\n",
    "c1r2,c2r2,c3r2,c4r2,c5r2\n",
    "c1r3,c2r3,c3r3,c4r3,c5r3\n",
];

my @store = map {(split /,/)[2]} @$lines;

local $, = ",\n";
print @store;
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