This might not be the best way, but it should work:

Update: It isn't the best way, isn't even a good way. See later in the thread for better solutions. Doh!

    my $file = "emails.txt";
    open(FILE, $file) or die "cannot open $file : $!\n";
    @content = reverse (<FILE>);
    close FILE;

print "<textarea cols=\"75\" rows=\"22\">";
foreach $address (@content) {
chomp($address);
print $address;
if ($address ne $content[-1]){print ", "}
}
print "</textarea>";
[download]

(In other words, print the comma unless its the last line). That assumes that each element in @content is unique. Given that you are using them for a mailing list, that should be a valid assumption. If not:

..
my $counter=0;
foreach $address (@content) {
chomp($address);
print $address;
$counter++;
if ($counter < @content){print ", "}
}
..
[download]

Set a counter and print the comma if the counter is smaller than the number of elements in the array.