#!perl -w

use strict;

my @lines;

while (<>) {
  push @lines, $_;
  shift @lines if @lines > 10;
}

chomp(@lines);

my @cols;
my $total = 0;

for (@lines) {
  @cols = split /:/;
  $total += $cols[3];
}
my $average = $total / (@lines || 1);
  # if no lines, avoid division by zero

print "Average of column 3 for the last ", scalar(@lines),
      " days: $average\n";