Don't. Please, if you're worried about the speed of one over the other, you're neck-deep in quibbling. Interpolation is comprised of the same op codes as concatenation, in the long run. Don't waste time with this.

And in the future, use Benchmark.

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Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;