The break-even point will vary among programmers. It will even for one programmer over time as you gain familiarity with both languages and the best practices for using both languages.
in reply to Re^2: From PHP to Perl - Should I, and how?
in thread From PHP to Perl - Should I, and how?
My rule of thumb these days for choosing between PHP and Perl is that if I find a well-designed, secure, fully functional open source project written in PHP for which there is no Perl equivalent, I'll customize the PHP project. If the available starting points are equivalent or I'm writing from scratch in either language, I always choose Perl over PHP. I learned Perl first and then PHP specifically to leverage existing code, so perhaps that has something to do with it.
I also use C, bash, Pascal, Forth, various forms of Basic, or Lisp for maintaining programs already written in those.
I sometimes use C, Forth, or Lisp just to keep my hand in and teach myself more about programming with them, as I don't use them very often. Those skills are too handy to let atrophy.