There's a subtle difference between the 2 forms you suggest: only the second works!

When you say

my ($x, $y, $z) = (0,0,0);
($x, $y, $z)=split;
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you always set $x, $y and $z! That's what assignment is all about!

Here it is in the debugger:

main::(-e:1):   0
  DB<1> ($x,$y,$z)=(2,3,4)

  DB<2> ($x,$y,$z)=split ' ',"two words"

  DB<3> x $x
0  'two'
  DB<4> x $y
0  'words'
  DB<5> x $z
0  undef

The second works, because it isn't trying to reassign undef.