You are getting there, but the way you were reading, chomping and splitting all at once caused a few problems, safer to break it up. Also, you weren't closing each file within the loop. Try this: 
for my $file (@files) {
    my $ok = open (my $fh, "<", "$dir/$file.txt");
    if (not $ok) {
        warn "Couldn't open file '$dir/$file.txt' ($!)\n";
        next;
    }
    while (my $line = <$fh> ) {    
        chomp $line;
        my @fields = split("\t",$line);
        my $key = $hash{"$fields[0].$fields[1]"} or next;
        $result{$key}{$file} += $fields[3];
    }
    close $fh;
}

my $hline = join("\t",@files);
print qq|\t$hline\n|;
for my $key ( sort keys %result ) {
    my $amt = $result{$key};
    my @cols = map { $amt->{$_} ? sprintf("%.1f",$amt->{$_}) : 'n/a' }
+ @files;
    my $line = join("\t",@cols);
    print qq|$key\t$line\n|;
}

[download]
Output: 
       file_1 file_2 file_3 file_4
AB     1.0    4.0    4.0    4.0
AB-1   2.0    6.0    6.0    6.0
AB-4   1.0    9.0    9.0    9.0
AB-6   2.0    8.0    4.0    4.0

[download]

In reply to Re: Perl - Hashes by tangent
in thread Perl - Hashes by Perlseeker_1

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