The traditional way is to use C, lex and yacc to write a cross-compiler, (tutorial: http://epaperpress.com/lexandyacc/) (where a browse through the theory chapters will help even when opting to use perl instead).

perl ought to make life easier, having several relevant CPAN modules to choose from instead, see lex and yapp A roadmap might be:

1) find a BNF for the COBOL or define one.

2) write a lexer using one of the lex modules

3) generate a parser using yapp

4) write a perl code generator to backend your parser.