Re^2: The sum of absolute differences in the counts of chars in two strings.

to put the whole thing into a single eval

You should. Try this, on my machine, this beats your grep solution:

sub usingtr_1e {
    eval join '+',
              map qq{abs(('$aa' =~ y/$_//)-('$bb' =~ y/$_//))},
                  'A' .. 'Z';
}
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