#!/usr/bin/perl
use strict;
use warnings;

my $list1 = 'jj@xyz.com,kk@xyz.com';
my $list2 = 'aa@xyz.com';

print join_email_lists( $list1,$list2),"\n"; #want comma separated
print join_email_lists( ''    ,$list2),"\n"; #no comma in front
print join_email_lists( $list1,''    ),"\n"; #no comma behind
print join_email_lists( ''    ,''    ),"\n"; #want ''

sub join_email_lists {
    my ($a1,$a2) = @_;
    return $a1 . ',', $a2 if $a1 and $a2;
    return $a1 if $a1;
    return $a2 if $a2;    
    return '';
}