Nope, no regexes for $/. If the file isn't too large or performance isn't too critical, you could do something like (untested, and requires 5.8.0):

open my $fh, "filename" or die $!;
while (my $line = <$fh>) {
   local $/ = "\r";
   open my $fh, "<", \$line or die $!;
   while (my $line = <$fh>) {
      # process $line here
   }
}
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Otherwise, open the file and read a bunch of characters (e.g. by setting $/=\1024; see perldoc perlvar) and check for \n or \r (e.g. with $/ = $1 if $buffer=~/([\r\n])/;)and set $/ appropriately; then seek to the beginning of the file and start the read loop.