Actually, it's trickier than that.  qr/(?<=\w)(?=\W)|(?<=\W)(?=\w)/ requires a character before and after the assertion, whereas  \b can match at the start and end of a string. A kind of double-negative is needed in an equivalent look-around:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'xx-xx';
 ;;
 my $lbw = qr/(?<=\w)(?=\W)|(?<=\W)(?=\w)/;
 printf qq{$-[0] } while $s =~ m{ $lbw }xmsg;
 print qq{\n};
 ;;
 my $wb = qr{ (?<!\w)(?!\W) | (?<!\W)(?!\w) }xms;
 printf qq{$-[0] } while $s =~ m{ $wb }xmsg;
 print qq{\n};
 ;;
 printf qq{$-[0] } while $s =~ m{ \b }xmsg;
"
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Update: Now,  \B is another story...