Contains the name of the program being executed. On some operating systems assigning to $0 modifies the argument area that the ps program sees. This is more useful as a way of indicating the current program state than it is for hiding the program you're running. (Mnemonic: same as sh and ksh.)
Note for BSD users: setting $0 does not completely remove "perl" from the ps(1) output. For example, setting $0 to "foobar" will result in "perl: foobar (perl)". This is an operating system feature.