And if you have Perl version 5.10 or greater, here's a variation that uses the  \K variable-width positive look-behind operator:

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my @lines = (
   'Once upon a time',
   'scrooge & donald',
   'went for a long walk',
   );
 ;;
 my @list = qw(huey dewey louis);
 ;;
 foreach my $who (@list) {
   print qq{who '$who'};
   foreach my $ln (@lines) {
     (my $newln = $ln) =~ s{ scrooge \s+ & \s+ \K donald }{$who}xmsg;
     print qq{    '$newln'};
     }
   }
"
who 'huey'
    'Once upon a time'
    'scrooge & huey'
    'went for a long walk'
who 'dewey'
    'Once upon a time'
    'scrooge & dewey'
    'went for a long walk'
who 'louis'
    'Once upon a time'
    'scrooge & louis'
    'went for a long walk'
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See  \K in Extended Patterns section of perlre. (Could also use a standard look-ahead to achieve the original  scrooge (& donald) match.)