Which version of AS Perl? It must be petty ancient as the last 7 or 8 version (at least) have been built with large file support. On win32 anyway. It's easy to forget that they also produce binaries for other OSs.

If you cannot upgrade for any reason, then I second the idea of using a system utility to read the file and pipe it into your script. I'd probably do it using the 'piped open'. If you need to re-write the data, send it to stdout and redirect the output via the command line.

die "You didn't redirect the output" if -t STDOUT;
open BIGFILE, "cmd/c type \path\to\bigfile |" or die $!;
while( <BIGFILE> ) {
    ## do stuff
}
close BIGFILE;
__END__
script bigfile.dat > modified.dat
[download]

Dying if STDOUT hasn't been re-directed is a touch that you'll appreciate after the first time you print a huge binary file to the console by accident. The bells! The bells! :)