use strict;
use warnings;
use Getopt::Long;

my $function;
my $data   = 'file1';
my $length =  4;
my $width  =  5;
my $verbose;

die "$0: Argument required.\n" unless @ARGV;

GetOptions
(
    'function=s' => \$function,
    'length=i'   => \$length, 
    'width=i'    => \$width, 
    'file=s'     => \$data,
     verbose     => \$verbose,

) or die "Error in command line arguments\n";

if    ($function eq 'plot')
{
    my $fh = read_fh($data);

    print "$_\n" while <$fh>;
}
elsif ($function eq 'calc')
{
    my $total = $width * $length;
    print $total;
}
else
{
    die "Unsupported function\n";
}

sub read_fh     # Open and Read a file
{
    my $filename = shift @_;
    my $filehandle;

    if ($filename =~ /gz$/)
    {
        open $filehandle, "gunzip -dc $filename |" or die $!;
    }
    else
    {
        open $filehandle, "<$filename" or die $!;
    }

    return $filehandle;
}

##</code><code>##

perl calc.pl --length 20 --width 30

##</code><code>##

perl main.pl --function calc --length 20 --width 30

##</code><code>##

...
my $plotlength = 10;
my $calclength = 20;
...

GetOptions
(
    ...
    'plotlength=i' => \$plotlength,
    'calclength=i' => \$calclength,
    ...

) or die "Error in command line arguments\n";