use constant USAGE => <<"EOT";
usage:
    perl $0 -yz input_file_name output_file_name
where:
    -y  optional: command line switch to enable Y
    -z  optional: command line switch to enable Z
    input_file_name   required: input file name
    output_file_name  required: output file name
    etc...
EOT
...
die USAGE unless @ARGV >= 2;
...