Be sure you don't measure the wrong thing. Read Chapters 21 (How Program Size Affects Construction) and 22 (Managing Construction) of Code Complete (ISBN ISBN 1556154844). It has pragmatical advice that should apply to all languages, even to Perl.

From Table 22.2, p. 545, "Useful Metrics":

Size

• Total lines of code written
• Total comment lines
• Total data declarations
• Total blank lines

Productivity

• Work-hours spent on the project
• Work-hours spent on each routine
• Number of times each routine changed
• Dollars spent on project
• Dollars spent per line of code
• Dollars spent per defect

There are also metrics listed for Defect Tracking, Overall Quality, and Maintainability, but I won't type those in here. Get the book and read it.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com