Lemme re-comment this for you:

use strict;
sub This::Does::Not::Fail { };

# Rightly so because you've already notified Perl that
# this is a subroutine name.  It *knows* this identifier
# is a sub.  Thanks for pre-declaring it!
This::Does::Not::Fail;                 # rightly so

# This fails because you've given perl no contextual hints
# that this is a subroutine.  It might be... a filehandle!
# Or something else.  But you haven't pre-declared it 
# either.  This is a bareword (an identifier).
This::Will::Fail;                      # as we expect

# You haven't pre-declared this thing.
# *BUT* you did give perl a contextual hint that this is 
# indeed a subroutine.  This is essentially a *promise* 
# you've made to strict that at runtime, when this code is
# encountered, you'll have a subroutine with this name
# all ready to go.
It::Bothers::Me::That::This::Works (); # wassup with THAT?!
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For that last point, there are other "promises" you can make and not just with parens. Some of these include:

$obj->method;   # Either $obj class' method will exist or
                # AUTOLOAD had better take care of it later.
$a=foo Bar;     # Bar will have a foo()!  Or AUTOLOAD...etc.
&foo;           # Again, you've promised it will exist...
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