You cannot (or rather, should not) use code as both, a function and a method. Using inheritance is the bad approach in any way.

If you want to call the subroutine in Person from Employee, just call it directly:

package Person;
my %WARN;
sub get_warnings {
    return \%WARN;
};
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package Employee;
use Data::Dumper;

sub frobnicate {
    my $warnings_by_person = Person::get_warnings;
    warn Dumper $warnings_by_person;
};
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