Hi there, it doesn't appear that you have formally declared the $pdbline variable. maybe trying

my $pdbline = <IN>;
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would work? But basically, you need declare the new variables with 'my'
or better yet, use a 3 pointed opening method like this:

my $filename = $ARGV[0];
open(my $fh, '<:encoding(UTF-8)', $filename)); 
while (my $pdbline = <$fh>) {
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