The GNU utility split can split on line counts, which seems to be very close to what you want - just divide by the number of chars per line (+1 for the line feed or +2 if using carriage return as well). See the split man page, option -l.

Update: Sorry - missed the criteria that each first line must begin with 100.

Untested and needs tidying:

use strict;
use warnings;

my $MAX_FILE_SIZE = 10_000_000_000;

my $num = 0;
my $next_outfile = sub {
    open my $OUT, '>', 'file_' . (++$num) or die $!;
    return $OUT;
}

my $OUTPUT;
my $curr_size;
my $process_chunk = sub {
    my $chunk = shift;
    if(not defined $curr_size 
        or $curr_size + length($chunk) > $MAX_FILE_SIZE) {
        $OUTPUT = $next_outfile->();
        $curr_size = 0;
    }
    $curr_size += length($chunk);
    print $OUTPUT $chunk;
};

my $chunk;
while(my $line = <INPUT>) {
    if($line =~ /^100/) {
        $process_chunk->($chunk);
        $chunk = '';
    }
    $chunk .= $line;
}
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Reading guide: code is best understood by starting with the while loop at the bottom.