#!/usr/bin/perl

use strict;
use warnings;

my @list = qw(a b c);
my %fh;
foreach my $f(@list){
        foreach my $s(@list){
                my $key = $f.$s;
                open($fh{$key}, ">", "fh_$key.txt") or die;
        }
}

while(<DATA>){
        chomp;
        my $sub = substr($_, 0,2);
        print  $fh{$sub} "$sub\n";
}

__DATA__
aaK
acK
bcK
baK
bbK