As a side note, it is interesting to see how List::Util deals with ($a,$b):

sub reduce (&@) {
  my $code = shift;

  return shift unless @_ > 1;

  my $caller = caller;
  local(*{$caller."::a"}) = \my $a;
  local(*{$caller."::b"}) = \my $b;

  $a = shift;
  foreach (@_) {
    $b = $_;
    $a = &{$code}();
  }

  $a;
}
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