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
}