Is your third argument a perl array, or are you thinking, C-like, of strings as arrays of characters?

For a perl array, you can pass an array slice,

format_address(':', 0, @array[8..13]);
[download]

To get an array of six single characters from position eight in a string,

my @array = split //, substr($scalar, 8, 6);
[download]

With Perl's string handling, it might be better to write the function to simply take a string.