chr    start    end    length
chr1   10       50      40
chr2    20      80      60
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In your OP, you show the following:

open (MYFILE, '148-N-pvalue0.01_peaks.xls');
#READ THROUGH THE FILE
   while (my $line = <MYFILE>)
   {
...
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opening an Excel spread sheet this way will not give you access to the data you want. If you're are trying to parse an Excel spread sheet, consider using a module, like Spreadsheet::ParseExcel for the job. Otherwise, you can use "Save As..." within Excel to save the data as tab-delimited in a text file, e.g., "148-N-pvalue0.01_peaks.txt", which you can then open and process as above.

toolic shows how to obtain the last column's average, which includes using Acme::Tools. Importantly, two other pragmas begin the solution:

use warnings;
use strict;
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Consider always beginning your scripts with these, as they'll preemptively catch any problematic areas in your scripts--likely saving you many headaches.