use strict;
use warnings;

use Carp;

...

my $dbh = DBI->connect(...parameters here...) or croak("$!");

...

my $sth = $dbh->prepare(...statement here...) or croak($dbh->errstr);

$sth->execute(...args here...) or croak($dbh->errstr);
my $mails = $query->fetchall_arrayref();
$sth->finish;