This problem is due to the prototype of the push function.

The prototype for push is \@@ and this means that perl needs to check that the first argument to push is an array.

You can make your own function that will give the same errors with

sub tst_push(\@@) { }
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Also note that the code below works, as perl can optimize out the ternary.

push(1 ? @a : @b, 'asdf');
push(1 == 0 ? @a : @b, 'asdf');
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The code above make it pretty clear to me that precedence is not the problem. If you don't like the push(@{$x ? \@a : \@b}.. syntax you could do this:

sub apush($@) {
    my $a = shift;

    push @$a, @_;
}
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and use

apush($cond>0 ? \@a : \@b, $elem);
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But to answer the question you pose: the push @{$x ? \@a : \@b}, $elem; code is portable over all perl versions.