use strict;
use warnings;

my $n = 'asdfklj  fuyy23 1234567890123456 78vc';

$n =~ s/(\d{6,6})(\d{6,6})(\d{4,4})/$1******$3/;
print "$1  $2  $3\n";
print $n;
__END__
123456  789012  3456
asdfklj  fuyy23 123456******3456 78vc