Since pod2usage by default exits, your explicit exit calls are redundant at best and confusing at worst. This

pod2usage(-verbose => 1) && exit if defined $opt_help;
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pod2usage(-verbose => 1) if defined $opt_help;
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You could also take advantage of the -message option. This

    print "$0: input_file required\n";
    pod2usage(-verbose => 0) && exit
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    pod2usage(-verbose => 0, -message => "$0: input_file required\n")
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A comment on Getopt::Long usage: instead of declaring several separate $opt_XXX scalar variables, it might be more natural Storing options values in a hash.