"below is the part of my code"

The fragment of your code that you've posted doesn't show the declaration (or subsequent assignment) of $hwwns. Look for that declaration in your code.

If you haven't declared that variable, the 'vars' stricture of strict would have advised you.

If you haven't used strict, you should have done.

Additional Information Update (I just noticed this after posting my response.):

Using strict would have also pointed out this:

(my $hwwnname, $hwwn) = split('= ', $ln);
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which you probably intended to be this:

my ($hwwnname, $hwwn) = split('= ', $ln);
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See the my documentation for the syntax for declaring a list of variables.