It is true that the trinary operator can be used as an lvalue. On the other hand, I can't think of a legitimate reason why one would want to do so (although I'm open to suggestions).

As a first approximation, I think the expression is better written as

  if( $flag ) {
    $on = $value;
  }
  else {
    $off = $value;
  }
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Then again, the repeated assignments of the same value does look a little redundant. This could be recast as

  ${$flag ? \$on : \$off} = $value;
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Which, not coincidentally, looks remarkably similar to the problem of pushing to the correct array, and to my twisted mind, seems a bit easier to follow than the initial code. The fact that the code looks weird alerts you to the fact that something weird is indeed going on.

But I digress. I posited that one should never use the ternary operator as an lvalue. When I see code that does this, it is usually a degenerate (in more ways than one) form of a hash. I would probably recast the two scalars as keys of a hash, which would give something like:

  $status{$flag ? 'on' : 'off'} = $value;
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That, to me, looks the clearest of all.