Deparse tells the tale... 
perl -MO=Deparse  -e 'sub Foo() {}; (time ? @a=() : $a) = Foo();'

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sub Foo () {
    
}
time ? @a = () : $a = Foo;

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and if you try the more natural: 
perl -MO=Deparse  -e 'sub Foo() {}; (time ? @a : $a) = Foo();'

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Assignment to both a list and a scalar at -e line 1, near ");"

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Perl needs to examine the left-hand side in order to know the context in which to run Foo(), but it delays execution until Foo() has been run. Therefore it can't decide on the context and fails.

In reply to Re: ternary operator as lvalue failing to induce evaluation of right-hand expression in list context by Loops
in thread ternary operator as lvalue failing to induce evaluation of right-hand expression in list context by ed_hoch

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