To expand on this, whenever you have a (update: just) variable in the substitution, it's as if you said /e. In a sense, then, /e just becomes syntactic sugar that you can do away with:

$ echo with bright knives he releaseth my soul | perl -wpe's/(\w+)/${\
+ucfirst($1)}/g'
With Bright Knives He Releaseth My Soul
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