You need to delay the evaluation. Perl calculates each expression as it runs it, so later changes will not change the values already computed:

#!perl -w
use strict;

my $ini=4;
my $last=2;
my %result=(
    SAS1=>$ini-$last, # 2
    SAS2=>$last-$ini, # -2
    SAS3=>$ini-$last  # 2
);
[download]

If you want to keep changing $ini and $last, you can do that in a subroutine:

#!perl -w
use strict;
use Data::Dumper;

sub get_SAS {
    my ($ini,$last) = @_;
    my %result=(
        SAS1=>$ini-$last, # 2
        SAS2=>$last-$ini, # -2
        SAS3=>$ini-$last  # 2
    );
    return %result
};

my %res42 = get_SAS(4,2);
print Dumper \%res42;
my %res105 = get_SAS(10,5);
print Dumper \%res105;
__END__
$VAR1 = {
          'SAS3' => 2,
          'SAS2' => -2,
          'SAS1' => 2
        };
$VAR1 = {
          'SAS3' => 5,
          'SAS2' => -5,
          'SAS1' => 5
        };
[download]

There are also other ways to delay the calculation, but I think this is the easiest approach.