Very nice code, much simpler & shorter than I expected. I tried to add an = (Perl or C internal-to-expression assignment) to the operators, though, lowest precedence & right-associative:

'='=> {prec=> 5,assoc=>'R',exec=>sub { print "DEBUG $_[0]=$_[1]\n"; $_[1]; }, function=>'asg'},

I added it to the operator pattern:

if (/\G\s*(\*\*|[=+\-*\/%\\])/gc)

and kind of defined the LHS variable

%var=(a=>101,b=>7,q=>'q');

but it fails because Function::evaluate tries to coerce the value, which I set to the variable name, into a number:

return $code->(map 0+$_,@{$self->{arguments}});

I think I have a workaround, but it looks like '=' is a special case because the LHS variable should not be evaluated before calling the assignment function.