Clever approach. Minor glitch in implementation.
Update: that should be "glitch in my interpretation." Neither Ratazong's nor OP's data is necessarsarily formatted as I did it.

my $first=0;

while (<DATA>) {
    chomp ($_);
    my $element = $_;
    if ($first == 0) { 
        $first = 1;   # don't print a comma before the first element
    } else {
         print ",";    # print a comma
    }
     print $element;
}

print "\n";

__DATA__
foo
bar
blivitz
this is a line
1
2
a
b
cde
[download]

Ratazzong produces:

foo
,bar
,blivitz
,this is a line
,1
,2
,a
,b
,cde
[download]

instead of (what I understand to be) OP's desire:

foo,bar,blivitz,this is a line,1,2,a,b,cde