There are more than one way to go old-school

 
use warnings;
use strict;


sub non_oxford_list {
    my ($s) = @_;
    my $p = rindex $s, ',';
    substr ($s,$p,1) = " and" if $p > -1;
    return $s;
}

use Test::More tests => 3;

is non_oxford_list('A'), 'A';
is non_oxford_list('A, B'), 'A and B';
is non_oxford_list('A, B, C'), 'A, B and C';
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FWIW: rindex , substr