To see this in action:

% perl -MO=Deparse -e 'use constant Y => "foo"; $x="bar"; return __PAC
+KAGE__, $x, Y'

  sub Y () {
      package constant;
      $scalar;
  }
  $x = 'bar';
  return 'main', $x, 'foo';   # <=== inlined values at compile time...
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Notice that __PACKAGE__ and the constant Y got inlined at compile time, whereas $x did not.

-Blake