While cryptic, and not exactly correct—see the UPDATE appended to my post before I received this reply, in short: there is no launching of a pager—this did get me to insert some print statements and compare the output in the two scenarios. The heart of the problem was indeed the lack of TTY, and a cryptic line from 8 years ago that I no longer grok the intent of: 
# This allows us to have multiple pseudo-STDOUT
return 0 unless -t STDOUT;

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Thanks!

--
In Bob We Trust, All Others Bring Data.

In reply to Re^2: Otherwise successful test fails under test_harness (-t) by belg4mit
in thread Otherwise successful test fails under test_harness by belg4mit

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