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my $string = "1234567890abcdefghijABCDEFGHIJK";
my $n = 2;    # $n is group size.
my @groups = unpack "a$n" x (length( $string ) /$n ), $string;

my @groups = unpack "a$n" x ((length($string)/$n)-1) . "a*", $string;