DB<234>  sub tst{
             my $href=shift;
             my @missing = grep { ! exists $href->{$_} } qw/name unit quantity/;
             print "Elements missing: @missing" if @missing;
           }

  DB<235> tst({name=>1,bla=>2,unit=>3,quantity=>4})

  DB<236> tst({name=>1,bla=>2,unit=>3})
Elements missing: quantity