average of column

[linseyr]

Hi, I would like to calculate the average of column 4 from my input files.

#!/usr/local/bin/perl
#OPEN UP THE FILE
open (MYFILE, '148-N-pvalue0.01_peaks.xls');
   #READ THROUGH THE FILE
   while (my $line = <MYFILE>)
   {
    @arr1 = split('\t',$line);
    @length = @arr1[3];
    $numbers = join(", ",@length);
    print $numbers;
    #print @length,",";
    #$avg = avg($length);
    
}

But the problem is the array. How do I get commas between the numbers? And at the end calculate the average from the array? Thanks!