You're right; I intended to link to perlsub. I still think that it's applicable, though I would be in full agreement that the documentation ought to be more explicit about it, unless specifying the behavior more explicitly locks future p5p development into a behavior that they would prefer to change.

Update:

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perlsub says this about implicit returns:

    If no return is found and if the last statement is an 
    expression, its value is returned. If the last statement is 
    a loop control structure like a foreach or a while , the 
    returned value is unspecified.

Now consider the following code:

    $x = 1;
    sub foo { if( $x ) { 0 } }
    print foo(), "\n";

The output will be 0, presumably because the last statement 
is the expression, numeric '0'.

How about this:

    $x = 0;
    sub foo { if( $x ) { 1 } }
    print foo(), "\n";

The output will be 0, presumably because the last expression
to be evaluated is '$x', which has a value of 0.  But the last
statement to execute is literally "if()".  If we run this past
B::Concise we find that the construct is converted into
something similar to sub { $x and 1 }, so it is intuitive that
the return value will be $x if false, or 1 if $x is true.

Although this behavior is possibly a little confusing to
someone who doesn't read between the lines in perlsub, it seems
reasonably stable (it's been with us forever), and unlikely to
change in the future.  Therefore, perlsub should state the
following:

    If no L<return> is found and if the last statement is an
    expression, its value is returned.  If the last statement
    is an C<if( CONDITION ) { BLOCK }> construct, the value
    of the return value will come from C<BLOCK> if C<CONDITION>
    is true, or from C<CONDITION> if C<CONDITION> is false.
    Relying on this behavior is detrimental to code legibility.
    If the last statement is a loop control structure like a
    C<foreach> or a C<while>, the returned value is unspecified.


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If it gains the consideration of p5p I anticipate they will want to rehash the wording a bit before proceeding, or possibly simply make the if(){} construct explicitly unspecified, as has been done with loop constructs.