A convenience variable which is typically used with SvPV when one doesn't care about the length of the string. It is usually more efficient to either declare a local variable and use that instead or to use the SvPV_nolen macro.

The input record separator - $/ in Perl space.

The GV which was last used for a filehandle input operation. (<FH>)

The output field separator - $, in Perl space.