Hmmm, the closest I could find is: 
B<NOTE:> The behaviour of a C<my>, C<state>, or  
C<our> modified with a statement modifier conditional
or loop construct (for example, C<my $x if ...>) is
B<undefined>.  The value of the C<my> variable may be C<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.

[download]
"Undefined behaviour", "may change in the future", "don't rely on it", and "here be dragons" are all hints not to use it. But it doesn't add up to "explicitly forbidden in the docs" in my book.

In reply to Re^4: order of operations with conditionals at the end by JavaFan
in thread order of operations with conditionals at the end by BeneSphinx

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