I think strong contenders for the top 5 are
DBI and
CGI. Scores of other popular modules build upon these cornerstones. Low level database access and the CGI protocol for webaccess are probably two things almost every programmer will need to do ever so often.
CountZero
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James
My blog:
Imperial Deltronics
