The caller can't tell the difference between a list of one item and a scalar, so you won't get perfect results. Maybe the following will do what you want, though:

sub determineType 
{
   my $sub_ref = shift;
   my @rval = $sub_ref(@_);
   return $rval[0] if @rval <= 1;
   return @rval    if @rval >= 2;
   my $rval = $rval[0];
   return @$rval if (ref($rval) eq 'ARRAY');
   return %$rval if (ref($rval) eq 'HASH');
}
[download]

By the way, you said "reference to a list", but there is no such thing. You meant "reference to an array".