DB<230>  sub tst{
             my $href=shift;
             my %needed;
             @needed{qw/name unit quantity/}=();
             delete @needed{ keys %$href };
             print "Elements missing; ", keys %needed if %needed;
           }

  DB<231> tst({name=>1,bla=>2,unit=>3,quantity=>4})

  DB<232> tst({name=>1,bla=>2,unit=>3})
Elements missing; quantity

  DB<233> tst({bla=>2,unit=>3})
Elements missing; quantityname