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

my @mails;
foreach my $userid (@userids) {
   push @mails, getMails($dbh, $userid);
}

sub getMails {
    my ($dbh, $userid) = @_;

    my $sth = $dbh->prepare("SELECT snd.EMAIL FROM FUSERS as m 
                             JOIN USERS as snd ON snd.USERID = m.USERID
                             WHERE m.USERID_IM = ?");
    $sth->execute($userid);
    my $mails = $sth->fetchall_arrayref();

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

##</code><code>##

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

my $sth = $dbh->prepare("SELECT snd.EMAIL FROM FUSERS as m 
                         JOIN USERS as snd ON snd.USERID = m.USERID
                         WHERE m.USERID_IM IN (?)");
$sth->execute(\@userids);
my $mails = $sth->fetchall_arrayref();

##</code><code>##

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;


##</code><code>##


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