use strict;

open FILE1, 'file1.txt' or die $!;
open FILE2, 'file2.txt' or die $!;
open NEW,   '>new.txt'  or die $!;

my %lookup;
while (<FILE1>) {
   my ($key,$id) = split(/\s*:\s*/,$_);
   $lookup{$key} = $id;
}

while (<FILE2>) {
   my @fields = split(/\s*:\s*/,$_);
   $fields[1] = $lookup{$fields[0]} || $fields[1];
   print NEW join(' :  ',@fields),"\n";
}