use warnings;
use strict;

my $chunk;
my $chunkSize = 40;
my $matchText = 'I';
my $partCount = 1;
my $fileSize = 0;

while (my $newChunkSize = read (DATA, $chunk, $chunkSize)) {
    next if $fileSize + $newChunkSize < $chunkSize; # not a complete chunk
    
    # Have at least $chunkSize - look for a stop point
    my $breakPoint = index $chunk, $matchText;
    if ($breakPoint == -1) {
        $fileSize += $newChunkSize;
        next; # Can't change file yet
    }
    
    # Found a stop point, all change
    print substr $chunk, 0, $breakPoint, '';
    
    # close current output file here and create the new file
    
    ++$partCount;
    print "\n\n***Start of new file for part $partCount\n";
    $fileSize = $newChunkSize - $breakPoint;
} continue {
    print $chunk;
}

__DATA__