It is a most unusual way to solve this kind of selection problems by using references. You get into the dreaded Action at A Distance anti-pattern by doing so. It is much more obvious to just write a function:

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;
    }
}
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Note that the sub compute_sas is sloppy, untested and probably not giving the right results, but it should illustrate my point.