It's often called the shebang line, and is mostly used on unix systems to tell the shell where it should look for the perl binary. Windows doesn't use this mechanism to find the binary, so that use is mute.

However, if the line is present, perl will inspect it and respect (some of) the runtime switches (see Perlrun).

I use it as a "reminder" for switches that I have permenantly enabled through the assoc/ftype mechanism (-sw) --mostly so people can see what I use when I post code.

I also use it to enabled individual switches on a case by case basis. The most frequent one being -l as it save me having to add "\n" to the end of every print line. Of course, there are times when you don't want a newline printed, in which case I just use printf $string; instead. Works for me:)