Just to pick a couple of nits in the interest of increasing understanding...

In that poem, the "m mmm" could be replaced with any bareword simply because the poem "won't run" and without strict, throwing around barewords is unlikely to cause compilation errors.

However, if "m mmm" was used in code to actually match something (being a silly way of writing "m//m"), then you could never replace it with "mmmm".

First, m//m is pretty silly code itself. The empty pattern means "reuse the most recent successful regex", and testing (since I so doubt it is documented that I didn't even look for it) shows that when using the empty regex this way, any options are ignored (including /g!); note that this might be considered a bug and get fixed some day, so don't rely on that behavior. So it boils down to being the same as m//, m//mg, m//g, or any other similar code.

But if we wanted to the code to compile to the same optree, then we could replace "=~ m mmm" with "=~ mmm" (note only 3 "m"s, not 4) provided we either didn't use strict or used a version of Perl that doesn't give an error in this case. However, in the absence of "=~", replacing "m mmm" with "mmm" will change the code from m//m (which matches against $_ since there is no "=~") to 'mmm' (which is simply a string constant) unless something else gets in the way such as strict or the declaration of a subroutine named "mmm".