The P65 assembler is an assembler for the 6502 microprocessor (such as is
used in the Commodore 64, Apple II, and Nintendo Entertainment System)
written entirely in Perl. It is designed to be able to support a wide variety
of output formats, since many people will be using this to produce code that
an emulator would like to see.

Features of P65 :
   - Highly portable
   - Flexible output format
   - Multiple file support
   - Temporary label support
   - Assemble-time expressions
   - Symbol table management
   - Optimal instruction selection

WWW: http://hkn.eecs.berkeley.edu/~mcmartin/P65/