sub format_output {
  my ($var1, $var2, @array) = @_;

  # do_some_stff
}

format_address(":", 0, (split(//, $scalar))[8 .. 13]);
[download]

sub format_output {
  my $var1 = shift;
  my $var2 = shift;
  my @arrayref = @_;
  do_some_stuff;
}

my $scalar = 'abcdefghijklmnopq';
my @array = ();
my @tmparray = split('',$scalar);
@array = splice(@tmparray, 8, 6);
&format_address(":", 0, @array);
[download]

I usually prefer to receive subroutine arguments with:

my ($var1, $var2, $parray) = @_;
[download]

In your original code, you've omitted the '$' sigil on your variables, which you need to supply.

Also, note that you can only pass a single array (as the last argument), because passing more than 1 will cause all arrays to be "flattened" into a single array.  If that isn't what you want (ie. if you need to pass more than 1), you can use references (I usually prepend 'p' to a variable used as a pointer/reference, like as parray):

sub format_output {
    my ($var1, $var2, $parray1, $parray2) = @_;
     
    # One way to iterate through an array
    foreach (@$parray1) {
        print "The next item in the array is $_\n";
    }

    # Another way to iterate through an array
    for (my $i = 0; $i < @$parray2; $i++) {
        printf "Array item #%d = %s\n", $i+1, $parray2->[$i];
    }
}
[download]

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]

sub format_output {
     my $var1 = shift;
     my $var2 = shift;
     my @array = @_;
     my $tmp = 0;
     for ($tmp = 0 ; $tmp <= $#array ; $tmp++) {
          do_good_stuff_to_array_elements;
     }
}

my @array = ();
# $scalar carries the binary data
@array = split('', $scalar);
&format_output(":", 0, @array[8 .. 13]);
[download]