The prepare/execute/fetch model is there because you will often want to prepare a statement once using placeholders, and then execute it several times using different data each time. (This is faster, and arguably more secure since it helps separate code from data)

my $sth = $dbh->prepare("insert into t (foo,bar) values (?,?)");
$sth->execute("foo", 1);
$sth->execute("bar", 2);
$sth->execute("baz", 3);
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Also, I tend to use the following loop a lot when fetching return data; it allows me to peek at the data and make certain changes that wouldn't have been efficient or possible on the DB server:

while (my $record = $sth->fetchrow_hashref) {

  # Manipulate record as needed
  $record->{'foo'} = &bar( $record->{'baz'} );

  push @records, $record;
}
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And voilá, I have an array of hashrefs. Very convenient.