#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11151901
use warnings;
use Path::Tiny;

local $_ = path('FILE1')->slurp . path('FILE2')->slurp;
my %seen;
while( /^(.*:)(.*)/gm )
  {
  $seen{$1}{$_}++ and print "$1$_\n" for split /~~/, $2;
  }