I ended up with ([^\-]). ... difference?

It depends on exactly what you want.  ([^\-]) requires a character match and consumes a character. As LanX pointed out,  ([^\-]|$) consumes a character if it can. Neither seem to do what you want:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'test atest atested tested test- -test test .test test. test'
+;
 ;;
 $s =~ s/\b(test)([^-])/DONE/g;
 print qq{'$s'};
"
'DONEatest atested DONEd test- -DONEDONE.DONEDONE test'

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'test atest atested tested test- -test test .test test. test'
+;
 ;;
 $s =~ s/\b(test)([^-]|$)/DONE/g;
 print qq{'$s'};
"
'DONEatest atested DONEd test- -DONEDONE.DONEDONE DONE'
[download]

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'test atest atested tested test- -test test .test test. test'
+;
 ;;
 $s =~ s/\b(test)(?!-)\b/DONE/g;
 print qq{'$s'};
"
'DONE atest atested tested test- -DONE DONE .DONE DONE. DONE'
[download]

Update: Just noticed from the timestamps that ultranerds already decided to go with LanX's suggestion. :)