s{<!-- \s ( . (?!-- \s* >) )* \s -- \s* >}{}gmsx

##</code><code>##

<!-- testing 
test--> 

##</code><code>##

#!perl

use 5.12.0;
use warnings;

{
    local $/ = undef;

    open my $fh, '<', $ARGV[0] or die $!;

    (my $html = <$fh>) =~ s{<!-- \s ( . (?!-- \s* >) )*  \s -- \s* >}{}gmsx;

    close $fh;

    say $html;
}

##</code><code>##

<!-- testing
test--> 

##</code><code>##

<!-- this is a comment -->
<!-- and so is this one,
    which occupies more than one line -->

##</code><code>##

s{<!-- \s ( . (?!-- \s* >) )*  \s* -- \s* >}{}gmsx