Because the docs says it isn't.

The behaviour of a my statement modified with a statement modifier conditional or loop construct (e.g. my $x if ... ) is undefined. The value of the my variable may be undef, any previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do something different from the version of perl you try it out on. Here be dragons.

( EmphasisStrong emphasis in original. )

That said, the current behaviour is actually quite predictable, and it doesn't limit $discard to the loop

>perl -cwe"use strict; my $x for 1..2; print $x"
-e syntax OK
[download]

So not only is not valid Perl, it doesn't even work.