while there's a next line
    extract the ID
    check if ID matches the previous ID
    if not:
        set previous ID to current ID
        increment a counter
        if counter > 100
            close current output file
            reset counter
    
    if output file isn't open
        open a new output file

    write the line to the current output file.

##</code><code>##

my $last_id;
my @group;
my $fh;
my $line_counter = 0;
my $file_counter = 0;

sub output {
    if ($line_counter + @group > 100) {
        $fh = undef;
        $line_counter = 0;            
    }

    if (!defined($fh)) {
        my $fn = sprintf('file%04d', $file_counter++);
        open($fh, '>', $fn)
            or die("Error create file $fn: $!\n");
    }

    $line_counter += @group;
    print($fh splice(@group));
}

while (<>) {
    my ($id) = /^(\S+)/;
    $last_id = $id if !defined($last_id);

    if ($id eq $last_id) {
        push @group, $_;
    } else {
        output();
    }
}

output() if @group;