... it also works with \b

... unless the string ends with the target character/pattern:

c:\@Work\Perl\monks>perl -wMstrict -le
"my @answer;
 'asdfgfhjkljx' =~ /f.*j(?{push @answer, $&})\b/;
 print for @answer;
"
fgfhjklj
fgfhj
fhjklj
fhj

c:\@Work\Perl\monks>perl -wMstrict -le
"my @answer;
 'asdfgfhjklj' =~ /f.*j(?{push @answer, $&})\b/;
 print for @answer;
"
fgfhjklj
[download]

Personally, I feel more comfortable using something that cannot possibly be true to force backtracking, like  (?!) or, from Perl 5.10 on,  (*FAIL) or  (*F) even though they take more keystrokes to type.

Update: Oops... I missed "... this special case (single word without a j at the end) ..." in Eily's post; see Eily's reply below.