Other than the inherent awesomeness of producing a Perl REPL in under 25 characters, I don't see why one would prefer this one-liner to the even shorter

perl -de1
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True, with the debugger one has to suffer the aggravation of having to prepend p or x to expressions if one wants to see the evaluated output, but in return one gets plenty of features like (off the top of my head):

1. Readline with history (this one alone saves so much typing that it more than compensates for having to type those extra p's and x's);
2. The option of paged output;
3. The Joy of x, including not only pretty printing of complex data structures (with optional paging), but also the ability to specify the depth the pretty-printing;