# A list of more than 1000 numbers separated by commas
my $list = '1444,99,112,1233,174,224,443,672,32,9821,...';

my $i = 0;
my @arr = split(',', $list);

# Move through the @arr copying elements to a temporary @subset array
# Do this in batches of 400
while ( $i le $#arr ) {
    my @subset;
    my $s = 1.0 * scalar(@subset);

    while ( $s le 400 ){
        push ( @subset, $arr[$i] );
        $s = 1.0 * scalar(@subset);
        $i++;
    }

    # Do something with the @subset array here
}