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my @userids = ('XD4555', 'JJKKKK');
...

    return @{$mails}; # Caller wants an array, so deref
}

my @userids = ('XD4555', 'JJKKKK');

...
                         WHERE m.USERID_IM IN (?)");
$sth->execute(\@userids);
my $mails = $sth->fetchall_arrayref();

use strict;
use warnings;
...
my $mails = $query->fetchall_arrayref();
$sth->finish;

if($dbh->do(...somestuff...) && $dbh->do(...otherstuff...)) {
...
    print STDERR "Database error: ", $dbh->errstr, "\n";
    $dbh->rollback;
}