in reply to Any caveats in using unpack to right-trim? Why isn't it advertised more?

Not sure why you mention trim, when what you show here is rtrim (which is not a builtin). When I added trim, it didn't perform that bad, especially if we consider it has to attempt to left trim, too: 
usual   290/s     --   -87%   -92%   -95%
better 2221/s   666%     --   -42%   -64%
trim   3829/s  1220%    72%     --   -38%
unpack 6164/s  2026%   178%    61%     --

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Also, the "better" substitution returns a different output for strings containing only whitespace.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In Section Seekers of Perl Wisdom