c:\@Work\Perl\monks>perl -wMstrict -le
"my $a = 'A_D1[1]';
 ;;
 my $str = '123 A_D1 XYZ A_D1[1] BLAH A_D1[1] BLAH';
 print qq{str = '$str'};
 ;;
 $str =~ s{ \b \Q$a\E [ ] }{-}xmsg;
 print qq{str = '$str'};
"
str = '123 A_D1 XYZ A_D1[1] BLAH A_D1[1] BLAH'
str = '123 A_D1 XYZ -BLAH -BLAH'