Another possible approach to doing what you request:

$ # Code (1-liner, expanded) and example output, 
$ #     handles both '###-##-####' and '#########' formats
$ perl -le '
my @c = qw/ 123-45-6789 123456789 / ; 
foreach my $d ( @c ) { 
    my $e = $d; 
    $e =~ s/^(\d+)(-?)(\d+)(-?)(\d{4})$/
        q{#} x length($1) . $2 . q{#} x length($3) . $4 . $5/egimsx; 
    print $e; 
}'
###-##-6789
#####6789
[download]

Hope that helps.