Hi,
Fletch is right, that it's usually not a good idea to do SGML/HTML/XML parsing using perl regex. But in case you really want to do it, the following snippet will remove the two tags, and the text in between.

 #!/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;
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