In order to do what I believe you intend, you'll need to use: 
@bar = (1,2,3) unless @bar;

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What is interesting is that this prints x: 
$bar ||= (1,2,'x');
print $bar, "\n";

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So perhaps the RHS is not being interpreted as a list constructor but as a sequence of expressions whose value is the last expression. That would explain this behavior: 
my ($a, $b);
($a, $b) ||= ('a', 'b', 'c'); # $a unchanged, $b <- 'c'
($a, $b) ||= (1, 2, 3); # $a unchanged, $b unchanged
$b = (1, 2, 'x'); # $b <- 'x'
$a ||= (3, 4, 5); # $a <- 5

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So it appears that ($a, $b) on the LHS is being interpreted as a sequence (not list constructor) of l-values whose value is the last (l-value) expression. However, in this case: 
($a, $b) = (7,8,9); # $a <- 7, $b <- 8

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we have list constructors on both the LHS and RHS.

In reply to Re: ||= oddity by pc88mxer
in thread ||= oddity by throop

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