I consider my example to pass $#a as an argument an extremely common case

While I disagree about the commonness of passing $#a, there are options if it is deemed to be too slow.

• $#a as an arg is an lvalue. Users can pass 0+$#a if they want the extra speed of an rvalue.

• $#a as an arg is an rvalue. Users can use ${ \$#a } if they want an lvalue.

• $#a as an arg is a delayed lvalue. I don't know if the mechanism can be extended to $#a, but hash element lookups are like this:

  $ perl -le'\$h{k}; print exists($h{k})?1:0'
  1
  
  $ perl -le'sub{ $x=$_[0] }->($h{k}); print exists($h{k})?1:0'
  0
  
  $ perl -le'sub{ $_[0]=$x }->($h{k}); print exists($h{k})?1:0'
  1
  
  $ perl -le'sub{ $x=$_[0] }->(@h{k}); print exists($h{k})?1:0'
  1
  
  $ perl -le'sub{ $_[0]=$x }->(@h{k}); print exists($h{k})?1:0'
  1
  [download]

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