I like the twisted logic but the best part is the bare $ at the end. I had to run it through B::Deparse to see what was happening:

    $ perl -MO=Deparse  -pe '$_=$'

    LINE: while (defined($_ = <ARGV>)) {
        $_ = $;
    }
    continue {
        die "-p destination: $!\n" unless print $_;
    }
[download]

It shows that in this case $ gets interpreted as $; the subscript separator. However, this means that your code inserts an extra character in the file as shown by this:     perl -pe '$.-32or$_=$' file | cat -A

But I'm still not sure why perl inserts that semicolon. It doesn't in other cases:

    $ perl -MO=Deparse  -pe '$_=$.'

    LINE: while (defined($_ = <ARGV>)) {
        $_ = $.
    }
    continue {
        die "-p destination: $!\n" unless print $_;
    }
[download]

Anyone have an explanation for this last point?

Update: blakem's answer below is right. This last case is a perl 5.005 issue with B::Deparse.

--
John.