I believe your problem is related to variable scoping. Try moving all your variables into your sub:

sub walk {
        my ($file, $prefix);
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Unrelated, but you probably want to escape the . in your regex because it has special meaning. Also, there is usually no need to quote a scalar variable:

        if ("$file"=~m/.nsf$/){
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should probably be:

        if ($file =~ m/\.nsf$/) {
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