print obscure('5555666677778888', 4, 2, '*');

sub obscure {
   my ($num, $tip, $tail, $char) = @_;
   my $repl_length = length($num) - $tip - $tail;

   substr($num, $tip, $repl_length) = $char x $repl_length;
   
   return $num;
}

__END__
5555**********88