DB<32> sub fu(&) {print shift}
  
  DB<33> fu sub {}
CODE(0x990f0a8)
  DB<34> $a= sub {}
  
  DB<35> fu $a #ERROR
Type of arg 1 to main::fu must be block or sub {} (not scalar dereference) at (eval 37)[/usr/share/perl/5.10/perl5db.pl:638] line 4, at EOF
  
  DB<36> fu \&$a #OK
CODE(0x997f168)