Nice -- it's a kind of clever inside-out way to say 
my $a if 0;
my $b = 3;

[download]
EDIT: As seen with this: 
$a=1;
$b=2;
sub x {
    my (0?$a:$b) = shift;
    print"a=$a\nb=$b\n";
    $a=shift
}
x(3,30);
x(4,40);
__END__
# OUTPUT:
a=
b=3
a=30
b=4

[download]

In reply to Re^4: my (0?$a:$b): a koan by educated_foo
in thread my (0?$a:$b): a koan by educated_foo

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