sub binSubst {
    my( $infile, $outfile, $regex, $repl, $maxlen, $bufsiz )= @_;
    binmode($infile);
    binmode($outfile);
    $bufsize ||= 16*1024;
    my $buf= '';
    # Read the next chunk, appending to any left-over bytes:
    while(  sysread( $infile, $buf, $bufsize, length($buf) )  ) {
        $buf =~ s/$regex/$repl/g;
        # How much to write out, unless...
        my $end= length($buf)-$maxlen;
        # ... we matched after that point and so
        # should write upto the end of last match:
        $end= $+[0]    if  $end < $+[0];
        # Write out what we can, removing it from the buffer:
        print $outfile substr($buf,0,$end,'');
    }
    # Write out any left overs:
    print $outfile $buf;
}