#!/usr/local/perl_ithreads/bin/perl

use strict;
use warnings;

use threads;
use threads::shared;

our %h = ( a => 1, b => 2, c => 3 );

share( $h{ a } );
share( $h{ b } );
share( $h{ c } );


our @locking_keys = qw/ a b /;


{
  # this loop introduces another block context
  lock( $h{ $_ } )  for @locking_keys;

  # here I wish to munge all of the lock
  # things at the same time, say copy data
  # from one to another.

  # this gives a warning, because the block
  # context of the above loop has already been
  # exited, causing the locks to be lost
  cond_signal( $h{ $_ } ) for @locking_keys;
}


{
  my @left_to_lock = @locking_keys;
  my $key;

  LOCK_LOOP:
    $key = pop @left_to_lock;
    lock( $h{ $key } );
  goto LOCK_LOOP if @left_to_lock;

  # now the things are really locked, and so no warning is issued here
  cond_signal( $h{ $_ } ) for @locking_keys;
}
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Edited: minor code fix ala ikegami's suggestion, and another comment to make it clear that I need all of the items locked simultaneously.