01  234567890123
perl -ne "$$_++||print" < infile > outfile
[download]

Very nice. ++

But it will fail if the infile looked like this: ;-)

    $ cat infile
    here
    comes
    trouble
    1
    2
[download]

--
John.

Have you tried your input? It works fine for me. As $_ isn't chomp()'d, it escapes the problem of modifying a readonly $1.

-sauoq
"My two cents aren't worth a dime.";

Here's a version which shouldn't rely on 5.8:

      01  2345678901234567
perl -ne '${_.$_}++||print'
[download]

-sauoq
"My two cents aren't worth a dime.";