%{} tries to dereference a hash ref.. which @_ certainly isn't.

my ($var, $val);
for (($var, $val) = each %{{@_ }}) {
    print "var = $var; val = $val\n";
}
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Note the double brackets; the inner pair builds an anonymous hash and returns a reference to it which the outer pair with % dereferences. However, you have a big pitfall in there in that duplicate keys will lead to loss of data - so this is not a universally useful solution. As japhy said: use splice.