#!/usr/bin/perl -p

use strict;
use warnings;
use vars qw(%var);

while ( /([A-Z][0-9]*)=(\S+)/g ) {
    my( $id, $val, $pos ) = ( $1, $2, pos );
    my $len = length $val;
    
    (my $tmp_val = $val) =~ s/([A-Z][0-9]*)/$var{$1}||''/ge;
    
    if ( $tmp_val =~ /^[\d\*\/\+\-\(\)]+$/ ) {
        $tmp_val = eval $tmp_val;
        $var{$id} = $val = $tmp_val unless $@;
    }
    
    substr $_, $pos - $len, $len, $val;
    pos = $pos - $len + length $val;
}

$ perl script.pl input.txt > output.txt
oder:
$ perl script.pl < input.txt > output.txt