Just the .pm files in the perl core distribution contain over 700,000 lines. You show that those, combined with a bunch of 3rd party modules, contain only around 100 instances of code likely to be able to make use of this proposed operator.
To be exact, in that amount of code I've found 77 instances.
And I have found 192 lines of code (two and a half times as much) which use the defined-or //=.
If anything I think you've shown that this is a very rare use case and hardly justifies its addition to core.
A bit more rare than the defined-or assignment operator //=, but anyways. Among 700.000 lines, 192 isn't that much either, so rareness of use cases isn't exactly a reason for an operator to be included or not.