Fixed it for you :)

#!/usr/bin/perl --

use strict;
use warnings;

use Net::IP;

my $ip; # only my, only $ip

if (@ARGV) {
    $ip = shift @ARGV;        
} else { 
    print "Enter an ip address: ";
    chomp( $ip = <STDIN> );
}

$ip = new Net::IP ( $ip )
    or die (Net::IP::Error());

print $ip->hexip(), "\n";
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