Not explicit enough. Perl is the only language I've ever seen where

  sub f { return 1, 2 }
  my %x = (a => f(), b => f());

produces a three-element hash that has '2' as a key and 'b' as a value with no warnings. "Even offsets are keys" is far more error-prone than an actual syntax for hash literals, and splicing a list (not a hash!) into a hash doesn't need syntax this terse because it's hardly ever what I wanted. I think even PHP is more sane here.