Personally I find the desire or need to temporarily comment out fair sized blocks of code is a warning of impending danger or difficulty. And usually these style of comments are meant to be temporary. Often the trouble is so imminent that it has already arrived.

Requiring commented code to be compilable seems like a feature to me. But I know what you mean.

Quick update: The following is not what Abigail said. Abigail pointed out that you can't comment the entire BEGIN block this way. You can't. You can't comment out any other subroutine definition either. Or forward sub declaration.

And you can comment out code in a begin block. Viz:

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

BEGIN { use constant COMMENT => 0; }

BEGIN {
if ( COMMENT ){
    print "silence\n";
}
    print "noise\n";
}
[download]