#!/usr/bin/perl

# Load data inside %times hash
my %times;
while (<DATA>) {
   next unless /\S/; # just in case...
   my ($month, $weeknum, $day, $val) = split /;/;
   $times{$month}{"wk$weeknum"}{$day} = $val;
}

# ... all your program here ...


__DATA__
May;1;Mon;24.50
May;1;Tues;23.40
May;1;Wed;22.40
May;1;Thu;25.01
May;1;Fri;24.10
... and so on...