I wonder if this might be a good time to use Perl's bizarre pass-by-reference semantics?

sub pad {
  my $char = pop;
  my $max = pop;
  
  $char = " " unless defined $char;
  $_[0] .= $char x ($max - length $_[0]);
}
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Now:

    pad($v, 30, 'x');    # $v is now 30 characters long
    print $v;
[download]