I seem to get the semicolon with your second case:

% perl5.6.1 -MO=Deparse  -pe '$_=$.'
   LINE: while (defined($_ = <ARGV>)) {
       $_ = $.;
   }
   continue {
       die "-p destination: $!\n" unless print $_;
   }

% perl5.8.0 -MO=Deparse  -pe '$_=$.'
   LINE: while (defined($_ = <ARGV>)) {
       $_ = $.;
   }
   continue {
       die "-p destination: $!\n" unless print $_;
   }
[download]

Update: Response to above update: with 5.00503 I get:

% perl5.00503 -MO=Deparse  -pe '$_=$.'
   LINE: while (defined($_ = <ARGV>)) {
       $_ = $.
   }
   continue {
       die "-p destination: $!\n" unless print $_;
   }
[download]

-Blake