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my @records = (
    { employee => 10001, form => 10, date => 20090101, },
    { employee => 10002, form => 10, date => 20080515, },
    { employee => 10003, form => 10, date => 20080323, },
);

sub summarize_by {
    my $field_name = shift @_;
...
    }
    return \%out;
}