#!/usr/bin/perl -w
use strict;

my %files;
my @infiles = qw(fileA fileB fileC);

for (@infiles) {
    open IN, "<", $_ or die "Cannot open $_:$!\n";
    chomp(@{$files{$_}} = <IN>);
    close IN;
}

open OUT, ">", "fileD" or die "Cannot open fileD:$!\n";

for my $line (0 .. $#{$files{fileA}}) {
    for my $file (@infiles) {
        print OUT $files{$file}[$line];
    }
    print OUT "\n";
}

close OUT;

##</code><code>##

$ cat fileD
111AAAaaa
222BBBbbb
333CCCccc