Something a bit simpler:

$ cat mask_digs.pl 
#!/usr/bin/perl
use strict;
use warnings;

while (<DATA>) {
    print $_;

    # Chop up the string
    my @list = map { defined($_) ? $_ : '' } split /(\d)/;

    # Replace all digits with X, other than first six and last 4.
    s/\d/X/ for @list[12 .. $#list-8];

    # Join it back together
    print join("", @list), "\n";
}

__DATA__
5432 1234 12346 6694
a16-994-332-1234-16
123321123333333
[download]

Running this gives me:

$ perl mask_digs.pl
5432 1234 12346 6694
5432 12XX XXXXX 6694

a16-994-332-1234-16
a16-994-3XX-XX34-16

123321123333333
123321XXXXX3333
[download]

Update: Oops, I posted version N-1, replaced. (Version N is commented, and preserves proper number of digits.)

...roboticus

When your only tool is a hammer, all problems look like your thumb.