Simple statements

     The only kind of simple statement is an expression evaluated
     for  its  side effects.  Every expression (simple statement)
     must be terminated with a semicolon.  Note that this is like
     C, but unlike Pascal (and awk).

     Any simple statement may optionally be followed by a  single
     modifier, just before the terminating semicolon.  The possi-
     ble modifiers are:


          if EXPR
          unless EXPR
          while EXPR
          until EXPR

     The if and unless modifiers  have  the  expected  semantics.
     The  while and until modifiers also have the expected seman-
     tics (conditional evaluated first), except when applied to a
     do-BLOCK or a do-SUBROUTINE command, in which case the block
     executes once before the conditional is evaluated.  This  is
     so that you can write loops like:

          do {
               $_ = <STDIN>;
               ...
          } until $_ eq ".\n";

     (See the do operator below.  Note also that the loop control
     commands  described  later  will NOT work in this construct,
     since modifiers don't take loop labels.  Sorry.)