use strict;
use warnings;
use DBI;

my $searchstring = "DROP TABLE mydata;";

$dbh = DBI->connect("dbi:Pg:dbname=$dbname", 'myuser', 'verysecret', {AutoCommit => 0}) or die("Can't connect to database");

my $sth=$dbh->prepare("SELECT * FROM mydata WHERE mycolumn = ?") or die($dbh->errstr);

if(!$sth->execute($searchstring)) {
    $dbh->rollback;
    exit 500; # or whatever you want to do in case of statement level error
}

while((my $line = $sth->fetchrow_hashref)) {
   # do something with the data
}
$sth->finish;
$dbh->rollback; # Since we didn't change anything, finish the transaction with a rollback instead of commit; just in case