sub _common_setvar {
    my $XX = shift;                             # XX=(db|cache)
    no strict 'refs';
    my $XX_prefix = $XX . '_prefix';
    debug("path_ops","_XX_prefix=%s",$XX_prefix);
    if (@_) {
      $$XX_prefix = $_[0];
      my $_XX_prefix_set  = "_" . $XX_prefix . "_set";
      debug("path_ops","_XX_prefix_set=%s",$_XX_prefix_set);
      $$_XX_prefix_set    = True;
      my $_save_XX_prefix = '_save' . $$XX_prefix;
      if ($_storage_dir_set) {
        my $tmp =
        $$_save_XX_prefix = _path_cat($storage_dir,$$XX_prefix);
        debugn("path_ops", 
          "cmn_sv: sd=%s, ddXX_p=%s, ddsave_XX_p=%s, ",
          $storage_dir, $tmp, $$XX_prefix);
        _dir_check($$_save_XX_prefix);
      }
    }
    $$XX_prefix;
  }
  
  sub db_prefix { 
    my $this=shift; 
    _common_setvar('db',@_);
    debug("path_ops","db=%s, sdb=%s",$db_prefix,$_save_db_prefix);
  }

  sub cache_prefix {
    my $this=shift; 
    _common_setvar('cache',@_);
    debug("path_ops","cache=%s, scache=%s",$cache_prefix,$_save_cache_
+prefix);
  }
[download]

Nasty.

You should have simply used a hash. Drop all the toxic $$XX stuff out. Use simple keys instead of complex variable names with symbolic refs on top.

$cache{'storage directory'}; for example.

Using hashes would prune your tree of subroutines, prevent them from having action at a distance, make it clear what variables are changing, and greatly improve your code readability.