Taint mode is like insurance. You may never need it, but it costs so little to keep it in, even after you believe you've tested all the possible input paths. Not only that, but future versions of Perl may decide that new things are leaks, and so your program will (rightfully!) bomb when Perl is upgraded. This has happened already, when all globbing in 5.004 was made "taint-unsafe" because it was discovered that it had always been unsafe to glob because of some bugs in the c-shell.

-- Randal L. Schwartz, Perl hacker