If you are trying to export a variable from your package, it needs to be declared as a "our" variable in that package to give it package scope, then you can export it.

use strict;   #update had left this off before 
use warnings;
package MY_package;
use vars qw(@ISA @EXPORT @EXPORT_OK %EXPORT_TAGS $VERSION);
use Exporter;
our $VERSION=1.0;
our @ISA = qw(Exporter);
our @EXPORT = qw($c_freq );

our $c_freq = 2;  #now $c_freq is visible as a global
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$Price_calc::amastringsentback, amastringsentback needs to be an our $amastringsentback. In this case you use the fully qualified name, so $amastringsentback wouldn't need to be exported, but it still does have to be an "our" variable (not a my variable) to get in the symbol table.

Update: as Anonymous Monk says undeclared var may be ok. I just never write code without strict and warnings - so didn't consider that point.