The ability to read, and manipulate Perl (the language) programmatically other than with perl (the application) was one that caused difficulty for a long time.
The cause of this problem was Perl's complex and dynamic grammar. Although there is typically not a huge diversity in the grammar of most Perl code, certain issues cause large problems when it comes to parsing.
Indeed, quite early in Perl's history Tom Christenson introduced the Perl community to the quote "Nothing but perl can parse Perl", or as it is more often stated now as a truism:
