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'

##</code><code>##

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"],
)