Also possible - but rude - is to put just a while(0) {} around it.

That will only work if what you comment 1) compiles and 2) doesn't contain any compile time effects (like BEGIN, 'use' or a subrouting declaration).

#!/usr/bin/perl 

use strict;
use warnings;

while (0) {
    sub foo {print "Hello, world\n"}
}

foo () if defined &foo;
__END__
Hello, world
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Abigail