use 5.014;

my $str = "123456";
say join '/' => $str =~ /../g;

my $val = "213450";
my ( $whole, $dec ) = $val =~ /(.*?)(..?)$/;
$dec = "0$dec" if length $dec == 1;
$whole =~ s/(.{1,3}?)(?=(...)+$)/$1,/g;
$whole //= 0;
say "$whole.$dec";