But what does your RegExp actually match? (Also a 'fixed' version based on what I think you think you want.)

c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = 'Three, Four, One, Two';
 ;;
 /([A-Z][a-z][.][\b])/;
 print qq{'$1'  '$2'  '$3'  '$4'};
 ;;
 /([A-Z] [a-z]+ (?: , | \b))/x;
 print qq{'$1'  '$2'  '$3'  '$4'};
"
Use of uninitialized value $1 in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value $2 in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value $3 in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value $4 in concatenation (.) or string at -e lin
+e 1.
''  ''  ''  ''
Use of uninitialized value $2 in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value $3 in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value $4 in concatenation (.) or string at -e lin
+e 1.
'Three,'  ''  ''  ''
[download]

Why does the first regex match nothing at all? (That should be fairly easy to answer: take a careful look at it.) Why does the second regex match something, but only once when you want it to match several times? Why does  'Three,' have a comma at the end? Do you really want to capture this character?

Update 1: Another thing to remember is that each successful regex match (and a  s/// substitution must do a match — and you're doing four  s/// in a row) that is executed "wipes out" all capture variables  $1 $2 $3 $n and only re-assigns those corresponding to an actual capture group in the latest successful match.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'foo bar baz';
 $s =~ m{ (foo) \s* (bar) \s* (baz) }xms;
 print qq{A:  '$1'  '$2'  '$3'};
 ;;
 $s =~ m{ (xyzzy) }xms;
 print qq{B:  '$1'  '$2'  '$3'};
 ;;
 $s =~ m{ (b \w*) }xms;
 print qq{C:  '$1'  '$2'  '$3'};
"
A:  'foo'  'bar'  'baz'
B:  'foo'  'bar'  'baz'
Use of uninitialized value $2 in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value $3 in concatenation (.) or string at -e lin
+e 1.
C:  'bar'  ''  ''
[download]

Update 2: Here's an approach (one of many) to the problem, but without the annoying  <STDIN> stuff:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'Three, Four, One, Two, xFive9';
 print qq{'$s'};
 ;;
 my @numbers = $s =~ m{ \b [[:upper:]] [[:lower:]]+ \b }xmsg;
 printf qq{'$_' } for @numbers;
 print '';
 ;;
 my %correct;
 @correct{ @numbers } = qw(one two three four);
 ;;
 my ($rx_search) =
   map  qr{ \b (?: $_) \b }xms,
   join '|',
   map  quotemeta,
   keys %correct
   ;
 print $rx_search;
 ;;
 $s =~ s{ ($rx_search) }{$correct{$1}}xmsg;
 print qq{'$s'};
"
'Three, Four, One, Two, xFive9'
'Three' 'Four' 'One' 'Two'
(?^msx: \b (?: Four|Three|Two|One) \b )
'one, two, three, four, xFive9'
[download]

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