Note that in the comments of the code examples in the Positive and Negative Look-behind sections, e.g.
    '(?<=this\ )that' # match 'that' when followed by 'this '
    '(?<!this\ )that' # match 'that' when NOT followed by 'this '
the word "followed" should be "preceded" in both cases, i.e.
    '(?<=this\ )that' # match 'that' when preceded by 'this '
    '(?<!this\ )that' # match 'that' when NOT preceded by 'this '
respectively.

Both the PRECEDING_PATTERN and the FOLLOWING_PATTERN are matched as the 'look-around' patterns, but neither are captured to a variable, only what is matched as part of your (PATTERN_OF_INTEREST) capture group ...

As an interesting (I hope) side note, a capture group embedded within a positive lookaround assertion will capture something:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'foobar';
 ;;
 print qq{positive look-behind capture: '$1'} if $s =~ m{ (?<= (foo)) 
+   bar   }xms;
 print qq{positive look-ahead capture:  '$1'} if $s =~ m{       foo (?
+= (bar)) }xms;
"
positive look-behind capture: 'foo'
positive look-ahead capture:  'bar'
[download]

An embedded positive look-ahead capture is a way to capture and extract certain overlapping matches:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = 'abcd efgh';
 ;;
 my @caps = $s =~ m{ \w+ }xmsg;
 dd 'non-overlapping captures: ', \@caps;
 ;;
 @caps = $s =~ m{ (?= (\w+)) }xmsg;
 dd 'overlapping captures: ', \@caps;
"
("non-overlapping captures: ", ["abcd", "efgh"])

(
  "overlapping captures: ",
  ["abcd", "bcd", "cd", "d", "efgh", "fgh", "gh", "h"],
)
[download]

Update: Slight, essentially trivial wording and emphasis changes.