I think the statement about "prepare(); loop { execute() }" being faster / more efficient than "loop { do() }" was meant to apply to this sort of comparison (untested):

use strict;
use DBI;
use Benchmark qw/timethese/;

my $times = shift;
my @vals = ( 10734..10845 );
my $sql = "UPDATE Tests SET foo = 'bar' WHERE ID = ?";

my $dbh = DBI->connect("connection_string...");
my $sth = $dbh->prepare( $sql );

sub dbi_do {
    for my $val ( @vals ) {
        (my $dosql = $sql) =~ s/\?/'$val'/;
        $dbh->do( $dosql );
    }
}

sub dbi_prep {
    for my $val ( @vals ) {
        $sth->execute( $val );
    }
}

timethese( $times, {
           DbiDo => \&dbi_do,
           PrepExec => \&dbi_prep,
          });
[download]

Of couse, that sort of demonstration also highlights a good reason, besides speed, for favoring the "prepare/execute" approach: using placeholders in the SQL statement tends to be better and easier than using quoted strings.

(updated to fix the "use Benchmark" line)