#!/usr/bin/perl
use strict;
use warnings;

my $string = <<EOFDATA;
whatever
<!-- REMOVESTART -->
test test test
<!-- REMOVEEND -->
you are trying to do
EOFDATA

my $start_tag = quotemeta('<!-- REMOVESTART -->');
my $end_tag = quotemeta('<!-- REMOVEEND -->');
$string =~ s{$start_tag.*?$end_tag\n*}{}gms;
print $string;