DBI does some tricky optimizations. You should consider that reference to be owned by the $sth so when you closed your database connection, it appears you also cleared your results. Try making a shallow copy of the data first and if that doesn't work, make a deep copy.

sub select_query {
    # shallow copy
    my @results = @{ $sth->fetchall_arrayref() || [] };
    return \ @result;
}

use Storable 'dclone';
sub select_query {
    # deep copy
    my @results = @{ dclone( $sth->fetchall_arrayref() ) || [] };
    return \ @result;
}
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