Why doesn't this already exist?

Because it's not as simple as it looks e.g

sub foo : inline {
  my %args = @_;
  ...
  return $something;
}
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This would require not just parsing out the sub but also the any munging of @_ and any exitting of the sub (e.g return, last etc). While I guess this could be done with a bit of op tree trickery or XS, a simple filter would only be suitable for dropping literal blocks of code (which would also have to comply with strictures). Having said that I probably will make a module, but with many, many disclaimers ;)
HTH

_________
broquaint