In this case, the warnings are wrong. Those variables *are* used more than once, but List::Util sets them in a fashion that Perl can't detect at compile-time. (Something like *{caller() . '::a' = ...;.) You can get rid of the warnings by including the following in your package:

$a = $a;
$b = $b;
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Ignore the warnings when trying to find your error, they are misleading you. (But do change the variable name in my $a = ...; to something else!)