Another version, using a tied hash:

#!/usr/bin/perl

package DynHash;
use Tie::Hash;
our @ISA = 'Tie::StdHash';

sub TIEHASH {
    my $class = shift;
    return bless {}, $class;
}
sub FETCH {
    my $self = shift;
    return ${ $self->{$_[0]}[0] } - ${ $self->{$_[0]}[1] }
}

package main;
use Data::Dumper;

tie my %result, "DynHash";

my $ini  = 4;
my $last = 2;

%result = (
    SAS1 => [ \$ini, \$last ],
    SAS2 => [ \$last, \$ini ],
    SAS3 => [ \$ini, \$last ]
);

print Dumper \%result;

$ini  = 10;
$last = 5;

print Dumper \%result;

__END__
$VAR1 = {
          'SAS3' => 2,
          'SAS2' => -2,
          'SAS1' => 2
        };
$VAR1 = {
          'SAS3' => 5,
          'SAS2' => -5,
          'SAS1' => 5
        };
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i.e., store references to $ini and $last in the hash, and dynamically recompute the difference from them at the time the values of the (tied) hash are being fetched.