passing hash to a function question

[Zysman, Roiy]

Hi all,
What is the meaning of the '+' sign before the shift command in my test sub.
Why doesn't it works without the '+' sign ?
10x Roy
 
test_sub({'key1'=>'value1','key2'=>'value2'});
 

sub test_sub{
   my %htest = %{+shift};   #why doesn't it works without the + sign
   print keys(%htest);
   print "end\n";
}