You should show the error message you are getting, as well as the exact command line you are using.

I have no problem when I use 's' to denote a string (Getopt::Long):

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

my %options;
GetOptions (\%options, 'IP=s');
print "$options{IP}\n" if exists $options{IP};
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Here are some sample command lines:

$ 901916.pl
$
$ 901916.pl -IP 10.34.34.456
10.34.34.456
$
$ 901916.pl -ip 10.34.34.456
10.34.34.456
$
$ 901916.pl -I 10.34.34.456
10.34.34.456
$
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