... zero-width look-behind ... has to involve a fixed number of characters to match ...

5.10 adds the  \K variable-width look-behind Special Escape (see  "(?<=pattern)" "\K" in the Look Around Assertions section of perlre):

>perl -wMstrict -le
"print qq{ver $]};
 for (@ARGV) {
   my $name = $_;
   $name =~ s{ \b Ma?c \K (\w) }{ \u$1}xmsg;
   print qq{'$_' -> '$name'};
   }
"  Mcdonald  Macdonald
ver 5.010001
'Mcdonald' -> 'Mc Donald'
'Macdonald' -> 'Mac Donald'
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