my $rez = join(",\n", @$user1, @$user2); # Remove the \n if you want
+ just CSV
[download]

this option just output this error

ARRAY(0x559d1462e390), ARRAY(0x559d1462e348)
[download]

You're just not quite doing enough de-referencing...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11153321
use warnings;
use DBI;
$SIG{__WARN__} = sub { die @_ };

my $dbname = '/tmp/d.11153321';
unlink $dbname;
my $dbh = DBI->connect_cached(
  "DBI:SQLite(RaiseError=>1,PrintError=>0):$dbname");
while()
  {
  my ($table) = split ' ', <DATA> // last;
  <DATA>;
  my (@fields) = split ' ', <DATA>;
  my $fieldnames = join ', ', map "$_ text", @fields;
  my $fields = join ', ', @fields;
  $dbh->do("create table $table ($fieldnames)");
  while( <DATA> )
    {
    /\S/ or last;
    my $values = join ', ', map "'$_'", split;
    $dbh->do("insert into $table ($fields) values ($values)");
    }
  }
system "sqlite3 $dbname .dump";

######################################################################
+#######

my $AID = "XD4555";

my $query = $dbh->prepare("SELECT
snd.EMAIL FROM
FUSERS as m
JOIN USERS as snd ON snd.USERID = m.USERID WHERE (m.USERID_IM = ?)");
$query->execute($AID);
my $user1 = $query->fetchall_arrayref();

my $query_other = $dbh->prepare("SELECT
snd.EMAIL FROM
FUSERS as m
JOIN USERS as snd ON snd.USERID = m.USERID_IM WHERE (m.USERID = ?)");
$query_other->execute($AID);
my $user2 = $query_other->fetchall_arrayref();

use Data::Dump 'dd'; dd {user1 => $user1, user2 => $user2};

my $TotalEmails = [ @$user1, @$user2 ];

use Data::Dump 'dd'; dd {TotalEmails => $TotalEmails};

for my $em ( @$TotalEmails )
  {
  my $rez = join ',', @$em;
  print "$rez\n";
  }

#my $TotalEmails = $user1 + $user2;
#
#foreach my $em ( $TotalEmails ) {
#    my $rez = join( ",", $em);
#    print "$rez\n";
#    }

__DATA__
USERS

ID USERID EMAIL
1  XD4555 JOHNE@DEO.COM
2  JJKKKK JANE@DEO.COM
3  JJKKKK JANE21@DEO.COM

FUSERS

ID USERID USERID_IM
1  XD4555 JJKKKK
2  JJKKKK TYYUPPO
[download]

Outputs:

PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE USERS (ID text, USERID text, EMAIL text);
INSERT INTO USERS VALUES('1','XD4555','JOHNE@DEO.COM');
INSERT INTO USERS VALUES('2','JJKKKK','JANE@DEO.COM');
INSERT INTO USERS VALUES('3','JJKKKK','JANE21@DEO.COM');
CREATE TABLE FUSERS (ID text, USERID text, USERID_IM text);
INSERT INTO FUSERS VALUES('1','XD4555','JJKKKK');
INSERT INTO FUSERS VALUES('2','JJKKKK','TYYUPPO');
COMMIT;
{ user1 => [], user2 => [["JANE21\@DEO.COM"], ["JANE\@DEO.COM"]] }
{ TotalEmails => [["JANE21\@DEO.COM"], ["JANE\@DEO.COM"]] }
JANE21@DEO.COM
JANE@DEO.COM
[download]

The Data::Dump output is there to see what the returned data really is. You could also use Data::Dumper, but I just prefer the ease of Data::Dump.