Try printing the value of '$location' within your loop, and surround it with some sort of visual delimiters. E.g.

while (my ($location, $devices) = each %devicecount) {
    
    # Test the above variables
    print "Loc: [[[$location]]]\tDev: [[[$devices]]]\n";

    foreach my $line (@workingdata) {
    ...
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It doesn't seem likely that it's actually undefined, since you can't have an undefined key in a hash. If you show us more of your code - especially a complete but minimal script that can be executed and demonstrates the error - we'd be in a much better position to help you.

Update: You should also make sure that the value of '$location' within the regex is treated as a literal string; this will prevent problems if it happens to contain any regex metacharacters. I.e.,

if (/\Q$location\ETEXT/) {
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-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells