Change that to:

open INFILE, "$inputfn" or die "Error reading input file: $!\n";
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open(INFILE, "$inputfn") || die "Error reading input file: $!\n";
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The problem is the precedence of || and ",". In your code, if open fails to open $inputfn for any reason, it tries to "open" the die statement! Use parens to say what you really mean, or use the ultra-low priority "or" operator, which makes the comma act as you might expect.

(Of course, this applies to both of your opens. When in doubt, use parenthesis around all your function calls.)