Here's an example of preparing DBI statements with variable numbers of placeholders. This is especially useful when writing a statement such as: 
SELECT *
FROM my_table
WHERE id_column IN (?, ?, ?, ?)

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where the number of placeholders is unknown at compile time.

Note that the first three occurences of @values are in a scalar context.

my @sths;
while (<>) {
  my @values = split ' ', $_;
  $sths[@values] ||=
    $dbh->prepare("SELECT * FROM my_table WHERE id_column IN (" .
                  join(', ', ('?') x @values) . ')');
  $sths[@values]->execute(@values);
  # ...
}

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