#!/usr/bin/perl

use strict;
use warnings;

my $file = 'test.txt';
my $content = do{ local (@ARGV,$/) = $file; <> };

my $orig = "## ----------------------
## muu
## ----------------------";

my $replace = "## Fisch
## Fleisch";

$content =~ s/\Q$orig/$replace/g;

print $content;