Depending on the number of rows, fetching them all can by quite memory consuming. My code (although for DBD::Pg) usually looks like this:

use DBI;
use Carp;
use English;
...
my $dbh = DBI->connect($self->{dburl}, $self->{dbuser}, $self->{dbpass
+word},
                               {
                                   AutoCommit => 0, # This is a proper
+ database, not a key-value store
                                   RaiseError => 0, # Child protection
+ mode off. We are doing our own error checking, thank you very much
                                   AutoInactiveDestroy => 1, # Just do
+ the right thing when using fork()
                               }) or croak("$EVAL_ERROR");
...
# sub bla($dbh) {
my $selsth = $dbh->prepare_cached("SELECT foo, bar, baz FROM bli WHERE
+ bla = ? ORDER BY blub")
        or croak($dbh->errstr); # If the statement is wrong, no point 
+of internal error handling

if(!$selsth->execute("foobar")) {
    $reph->debuglog($dbh->errstr);
    $dbh->rollback;
    return 0; # Tell caller we failed
}

# Work on the data row-by-row
while((my $line = $selsth->fetchrow_array)) {
    ...
    # Do something with the data
    ...
}

$selsth->finish; # Make sure we don't leave database pointers and stuf
+f around

$dbh->commit; # Finish the current transaction, unlock all locked tabl
+es/rows and stuff.
              # commit() may or may not happen in THIS function, depen
+ding on if the caller needs to
              # chain multiple functions in a single transaction


return 1; # tell caller we succeeded.
...
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