First, a little background info. There are two major contexts we are concerned with here: scalar and list (the others are boolean, void and interpolative -- but that's another topic). There is no hash context. wantarray will tell you if the context is scalar or list, but not if you want a hash.

That said, you can only tell if you want one value or a list of values. For example:

$var = mysub();
@var = mysub();
%var = mysub();

sub mysub {
    print "You want a scalar\n" unless wantarray;
    print "You either want a hash or an array\n" if wantarray;
}
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One quick way I can think of to get around this is by passing references. For example,

my($var,@var,%var);

&mysub(\$var);
&mysub(\@var);
&mysub(\%var);

sub mysub {
    my($ref) = shift;
    print "You want a scalar\n" if ref($ref) eq 'SCALAR';
    print "You want an array\n" if ref($ref) eq 'ARRAY';
    print "You want a hash\n"   if ref($ref) eq 'HASH';
}
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Hope that helps,
Shendal