The issue is that keys requires a hash (or an array), but you are (attempting to) provide it a bunch of scalars.

But while keys doesn't help, working with such data is exactly the purpose of the pair* functions in core module List::Util.

say for pairkeys f;
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You could also use grep, but it's a bit inelegant.

sub pairkeys { my $i = 0; grep { $i ^= 1 } @_ }
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