By default, Getopt::Long does not check if you didn't specify an option. From the POD:

Options are not necessary for the program to work, hence the name 'option'

Since you require the input file --in option, you need a check to see if the user specified it:

pod2usage(-verbose => 1) unless defined $infile;
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Another possibility is to provide a default value for $infile in your code.