my $last = 0;
my $ini = 0;

...

$last = <something else>;

...

$ini = <something else>;

... # now you need $some_value:

my $selector = 'SAS1';
my $some_value = compute_sas($selector, $ini, $last);

sub compute_sas {
    my($selector, $ini, $last) = @_;

    if($selector eq 'SAS1') {
        return $ini - $last;
    } else {
        return $last - $ini;
    }
}