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

>perl -wMstrict -le
"my $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????????'