use strict;
use warnings;
use feature 'say';

sub mixedregex {
  my $foo = shift;
  say length $foo;
  $foo =~ s{\0([0-7]{3})}{chr oct $1}eg;
  say length $foo;
  return $foo;
}

say mixedregex("bar"."\0"."123");

__END__

##</code><code>##

$ perl 11142131.pl
7
4
barS