$GLOBAL1 = do_stuff($GLOBAL0, $GLOBAL1);

##</code><code>##

... set globals ...
...
do_stuff;
...
... use modified globals

##</code><code>##

&read_input;
&calculate_this;
&calculate_that;
&write_output; # some to files others to databases
               # all in one function.

sub calculate_this { ... }
sub read_input { ... }
sub write_output { ... }
sub calculate_that { ... }