Its normally helpful if you post your code and describe what you are having difficulty with. But I'm at a loose end so try:

#!/usr/bin/perl
use warnings;
use strict;

open (IN, 'first.txt') or die ("crap! $!");
my @first = <IN>;
close IN;

open (IN, 'second.txt') or die ("crap! $!");
my @second = <IN>;
close IN;

my %h;
my $i=0;
for (@second) {
    ($h{'A'},$h{'B'},$h{'C'}) = split / /,$_;
    my @keys = split //,$first[$i];
    my $total = 0;
    for(@keys) {
            $total+= int($h{$_}) unless /\n/;
    }
    $total/=scalar(@keys) if($total);
    print "$i: $total \n";
    $i++;
}
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