I remember this particular factiod because it has bothered me in the past that the return value of if isn't more clearly documented

Because it's not an expression. Expressions evaluate to a value, not statements. Nothing ever wants the value of an if statement.

Quote perlsub (for subs, but the same applies to do BLOCK):

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. The empty sub returns the empty list.

An if statement isn't a "loop control structure", but more importantly, it's not an expression either. One shouldn't place an if statement as the last statement of a block from which we expect a value.

That said, people do use if statements that way, and the result has been quite predictable. (Update: Mostly predictable. Exception) The last expression evaluated during the course of the processing of the if statement is the resulting value. 

my $x = 1;
if ($x) {
   f();      # f() is the last expression evaluated.
} else {
   g();
}

[download]
my $x = 0;
if ($x) {
   f();
} else {
   g();      # g() is the last expression evaluated.
}

[download]
my $x = 0;
if ($x) {    # $x is the last expression evaluated.
   f();
}

[download]
my $x = 0;
if ($x == 1) {
   f();
} elsif ($x == 2) (    # $x==2 is the last expression evaluated.
   g();
}

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The same goes for while statements, until statements, and bare loops ({ }). Foreach loops are obviously different (since the iterator is hidden from us).

Updated

In reply to Re^4: printing unitialized value of the 'do BLOCK' by ikegami
in thread printing unitialized value of the 'do BLOCK' by rsFalse

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