This is one of my unfavorite details. An lvalue-d sub doesn't work quite right with a regex:

{
  my $s;
  sub str :lvalue { $s }
}

str = "japhy";
pos(str) = 2;
print pos(str);               # prints 2!
str =~ /\G(.)/ and print $1;  # prints 'j', not 'p'
print pos(str);               # prints 2!
[download]

Maybe I should perlbug this...