D'oh! Reading your comment reminded me of the solution--using "our"--that I had found earlier but misremembered as using "local". Thanks much. 
# Works.
my $a = 'whatever'; # just happens to be in scope of below sort block
@arr2 = sort { our $a; $a <=> $b } @arr1;
print join(' ', @arr2), "\n";

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In reply to Re^2: my $a outside sort block incompatibility by Anonymous Monk
in thread my $a outside sort block incompatibility by Anonymous Monk

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