That reminds me of a very useful bit of information reguarding locking. A single lock variable can be used to control access to multiple shared variables. For example,

my $list_head : shared;   # Access controlled by $list_head.
my $list_tail : shared;   # Access controlled by $list_head.

sub ... {
   ...
   {
      lock($list_head);
      ... code that uses $list_head and/or $list_tail ...
   }
   ...
}
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It doesn't matter which variable is used as to control access to a give shared variable, as long as you *always* use the same lock variable to control access to that shared variable.