For production code, I think I prefer something like rjt's approach, but here's a cute variation:

>perl -wMstrict -le
"my $s = '---------xxxxxxxxxxxxxxx------yyyyyyyyyyyyyy--------';
 print qq{'$s'};
 ;;
 $s =~ s{ (?: \G | (?= -+ \z)) - }'?'xmsg;
 print qq{'$s'};
"
'---------xxxxxxxxxxxxxxx------yyyyyyyyyyyyyy--------'
'?????????xxxxxxxxxxxxxxx------yyyyyyyyyyyyyy????????'
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