I'd write that more concisely but essentially the same way. defined($pad_char) is preferred because the pad string might '0'.

sub my_pad {
  my ($s, $max_length, $dir, $pad_char) = @_;

  my $pad_string = (defined $pad_char ? $pad_char : ' ' ) x ($max_leng
+th - length $s);
  return $dir eq 'R' ? $s . $pad_string : $pad_string . $s;
}
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In perl 5.10, it's shorter because of the new err operator. That's an import from perl 6.