The reason, of course, is Perl's prototyping behavior. Example:

sub proto ($@) {
  print "$_[0]; $_[1..$#_]\n";
}

my @args = qw( a b c d e f );

proto($args[0], @args[1..$#args]);
proto(@args[0..$#args]);
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Perl applies scalar() to the first "argument" you pass proto() at compile-time. In this case, that's $args[0] and @args[0..$#args] -- in the former, there is no change, but in the latter, we end up getting the last element in the array! When you pass an array as the first element, it gets transmuted into its size. (But you probably knew most of this already.)

I've been in a similar situation. The simplest work-around I can think of is the one you've shown, where you separate the first element of the array from the others. Whether you do that destructively (as with shift) or with slices is a personal choice.