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Processors are typically designed in Register Transfer Level (RTL) languages, which give designers low-level control over circuit structure and timing. To achieve good performance, processors are pipelined, with multiple instructions executing concurrently in different parts of the circuit. Thus even though processors implement a fundamentally sequential specification (the instruction set architecture), the implementation is highly concurrent. The interactions of multiple instructions---potentially speculative---can cause incorrect behavior. We present PDL, a novel hardware description language targeted at the construction of pipelined processors. PDL provides one instruction at a time semantics: the first language to enforce that the generated pipelined circuit has the same behavior as a sequential specification. This enforcement facilitates design-space exploration. Adding or removing pipeline stages, moving operations across stages, or otherwise changing pipeline structure normally requires careful analysis of bypass paths and stall logic; with PDL, this analysis is handled by the PDL compiler. At the same time, PDL still offers designers fine-grained control over performance-critical microarchitectural choices such as timing of operations, data forwarding, and speculation. We demonstrate PDL's expressive power and ease of design exploration by implementing several RISC-V cores with differing microarchitectures. Our results show that PDL does not impose significant performance or area overhead compared to a standard HDL.