s/// replaces everything that matched on the left with whatever matched on the right. What you want to do is store which one of [c|d] matched, and then put that back into the match, ala

$bleh="-a 123 -b 456 -c 789 -d 333"; 
$bleh=~s/-([bc])/-:$1/g;
print $bleh;
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The $1 evaluates to whatever ([bc]) matched (either b or c) and then replaces everything matched on the left with whatever is appropriate on the right.