# ... stuff run every time this script is invoked
   if [ "$checkvar" != "foo" ]; then
     exec perl /usr/script/myscript.pl
   else
     # ... stuff to do after the script has run
   fi

##</code><code>##

  $ENV{checkvar}="foo";
  exec "sh", "/usr/script/myshell.sh" # or, if you're logging in
  exec "sh", "-l"