my $str = join ', ', map { "'$_'" } split /,/, "CAT,DOG,BIRD,COW";     # -> 'CAT', 'DOG', 'BIRD', 'COW'

##</code><code>##

my $str = "CAT,DOG,BIRD,COW";
$str =~  s/\b/'/g;     # -> 'CAT','DOG','BIRD','COW'