... is there a way to require a reference in the signature?

The  \@ \% \& prototypes cause references to the given types of data to be taken and passed to a subroutine. In a sense, you can say this is "requiring a reference."

c:\@Work\Perl\monks>perl -wMstrict -le
"sub foo (\%\@) {
   my ($hashref, $arrayref) = @_;
   ;;
   print ref $hashref;
   print ref $arrayref;
   ;;
   print qq{spanish for 'two' is '$hashref->{two}'};
   print qq{@{ $arrayref }};
   }
 ;;
 ;;
 my @ra   = qw(fee fie foe fum);
 my %hash = qw(one uno  two dos  three tres);
 ;;
 foo(%hash, @ra);
"
HASH
ARRAY
spanish for 'two' is 'dos'
fee fie foe fum
[download]

(See what happens if you use a hash in place of an array, etc.)

Of course, in a call to a "normal" (i.e., unprototyped) function like  no_proto(%hash, @array) all the elements of the hash(es) and array(s) would be "flattened" in the function argument list, and there would be no way of telling where one left off and the next began.

I use subroutine signatures when I can ...

I can understand why someone who comes from a background in C, C++ or other strongly typed language would feel very comfortable with Perl Prototypes, and think that they understood them as soon as they saw them. I certainly did. But Perl prototypes are a different beast, and in time you may see your sense of understanding and comfort melt away. Please see Far More than Everything You've Ever Wanted to Know about Prototypes in Perl -- by Tom Christiansen for a number of reasons not to use prototypes on a regular basis, but only in rather specialized circumstances — where they can, indeed, sometimes be very useful!