1:2:3:4
5:6:7:8
:9:10:11
12:13:14:15
:17:18:19

##</code><code>##

a:b:c:d:e
f:g:h:i:j
k:l:m:n:o
p:q:r:s:t
u:v:w:x:y

##</code><code>##


use File::Copy;
use strict;
my (@col, $val, $i);
open(COL, "<replace.txt");

while(<COL>) {
    push @col, $_ =~ m/(.*?):/;
}
close(COL);

open(OUT, ">out.txt");
open(COL, "<original.txt");
while(<COL>) {
    ($val) = $_ =~ m/(.*?):/;
    $_ =~ s/(.*?)(?=:)/$col[$i++]/ unless $val =~ m/^\s*$/;
    print OUT $_;
}
close(COL);
close(OUT);
copy("out.txt", "original.txt");

##</code><code>##

a:2:3:4
f:6:7:8
:9:10:11
k:13:14:15
:17:18:19