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$_       = 'foo bbbbbbbah bar';
$find    = 'bah';
$replace = 'ah';

$_       = 'foo babab bar';
$find    = 'bab';
$replace = 'xxx';

sub replace {
   die "Some helpful messsage" unless 3 == grep defined $_, @_[0..2];
...
   substr($str,$_,$l,$rep) for reverse @pos;
   return $str;
}