You can achieve this using tie on the hash entries you wish to be dynamic:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];

{
    package Tie::Scalar::Dynamic;
    sub TIESCALAR {
        my( $class, $coderef ) = @_;
        return bless $coderef, $class;
    }

    sub FETCH {
        my $this = shift;
        return $this->();
    }

    sub STORE {
        die 'This tied variable is readonly';
    }

}

my( $ini, $last ) = ( 4, 2 );

my %result = (
    SAS1 => 0,
    SAS2 => 0,
    SAS3 => 0,
);

tie $result{ SAS1 }, 'Tie::Scalar::Dynamic', sub{ $ini - $last };
tie $result{ SAS2 }, 'Tie::Scalar::Dynamic', sub{ $last - $ini };
tie $result{ SAS3 }, 'Tie::Scalar::Dynamic', sub{ $ini - $last };

pp \%result;

( $ini, $last ) = ( 123, -456 );

pp \%result;

__END__
c:\test>835979.pl
{ SAS1 => 2, SAS2 => -2, SAS3 => 2 }

{ SAS1 => 579, SAS2 => -579, SAS3 => 579 }
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