Parse::Flex works similar to Parse::Lex, but it uses XS for faster performance. This module allows you to construct a lexer analyzer with your custom rules. Parse::Flex is not intended to be used directly; instead, use the script makelexer.pl to submit your grammar file. The output of the script is a custom shared library and a custom .pm module which, among other things, will transparently load the library and provide interface to your (custom) lexer. In other words, you supply a grammar.l file to makelexer.pl and you receive Flex01.pm and Flex02.so . Then, use only the Flex01.pm - since Flex01.pm will automatically load Flex01.so. The grammar.l file requires the same syntax as flex(1); that is, the actions are written in C . See the flex(1) documentation to learn the syntax, or fetch the sample t/grammar.l file inside this package. WWW: https://metacpan.org/release/Parse-Flex