How big of a CSV are we looking at here? If you have the memory, you could simplify with $sth->fetchall_arrarref({}) which would give you something like this:

$VAR1 = [
    {
        'id' => 1,
        'fname' => 'John',
        'lname' => 'Smith',
    },
    {
        'id' => 2,
        'fname' => 'Sally',
        'lname' => 'Smith',
    },
];
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Then with your friend map, you can massage this structure thusly:

my %hash = map { $_->{'id'} => $_ } @$data;
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which would result in something along the lines of

$VAR2 = {
    '1' => {
            'id' => 1,
            'fname' => 'John',
            'lname' => 'Smith',
        },
    '2' => {
            'id' => 2,
            'fname' => 'Sally',
            'lname' => 'Smith',
        },
};
[download]