One way would be to do something like this:

open(FILE,'41084109.udp');
$/=\4;
$a = <FILE>;
$b = <FILE>;
[download]

The $/ variable determines how much to read in. In this case its set to 4, to read in 4 bytes (its default value is "\n").

This might be more useful if you need more than two sets of four numbers, for something like:

open(FILE,'41084109.udp');
$/=\4;
while(<FILE>) {
     #count the occuarnce of the 4 digits
     $count{$_}++;
}
[download]

{QA Editors note: The original question's subject line was somewhat ambiguous as to whether the person asking was trying to split a file, or a filename. The followup text was still somewhat unclear, but the example shown by the original poster clarified that he was trying to split a filename. This response will aid in splitting the contents of a file, which is different but worthy of mention.}

Originally posted as a Categorized Answer.