#!/usr/bin/perl

use strict; # https://www.perlmonks.org/?node_id=11155633
use warnings;

use Data::Dump 'dd';
for ( 'aa', "a\xff", 'frequency/is/measured/in/hertz' )
  {
  dd 'in', $_;
  my $onelarger = s/([^\xff])\xff*\z/ $1 =~ tr||\x01-\xff|cr /er;
  dd 'out', $onelarger;
  }

##</code><code>##

("in", "aa")
("out", "ab")
("in", "a\xFF")
("out", "b")
("in", "frequency/is/measured/in/hertz")
("out", "frequency/is/measured/in/hert{")