For something like this, I would probably inspect the @ARGV array and call a usage() subroutine if:

• @ARGV was empty
• a file test like '-e' failed on the first element of @ARGV
• I was unable to open the file specified in @ARGV

Update: Dang! Beat to the punch by ikegami! :)

A sample usage() subroutine might look like this:

sub usage {
  die "You must specify an input file.\nUsage: $0 <file_name>\n";
}
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