Perl Script Session values

[Crysallis]

I can't seem to locate an answer to this.. but it seems like this should work.. 

my $DiskMBFreeThreshold;
$DiskMBFreeThreshold = 100;
$Session->Value('DiskMBFreeThreshold', $DiskMBFreeThreshold);

my $ExcludeFilter;
$ExcludeFilter = "[D:|E:|F:|G:|K:|L:|M:|N:|O:|P:|Q:|R:|S:|T:|U:|V:|W:|X:|Y:|Z:]";
$Session->Value('ExcludeFilter', $ExcludeFilter);

So I have these two values... when I attempt to use the text based $Session->Value('ExcludeFilter'); I'm told that

Perl Script execution failed: (in cleanup) Can't call method "Value" on an undefined value at PerlScript line 39.

but the numeric variable works fine.  

This is a threshold policy and I'm setting that value using the parameters...and calling it on an instance filter.. 

$ExcludeFilter = $Session->Value('ExcludeFilter'); <-- this is line 39

Any ideas?