$string =~ s/\A(-+)|(-+)$/("?" x length $1).("?" x length $2)/eg;
[download]

That does the substitutions, but will also produce warnings if run with warnings:

>perl -wMstrict -le
"my $s = '---------xxxxxxxxxxxxxxx------yyyyyyyyyyyyyy--------';
 print qq{'$s'};
 ;;
 $s =~ s{ \A (-+) | (-+) \z }
        { ('?' x length $1) . ('?' x length $2) }xmsge;
 print qq{'$s'};
"
'---------xxxxxxxxxxxxxxx------yyyyyyyyyyyyyy--------'
Use of uninitialized value $2 in repeat (x) at -e line 1.
Use of uninitialized value $1 in repeat (x) at -e line 1.
'?????????xxxxxxxxxxxxxxx------yyyyyyyyyyyyyy????????'
[download]