Not beautiful, but better, I hope:

#!/usr/local/bin/perl
use strict;
use warnings;
use Term::ReadKey qw(ReadMode ReadLine);
use IPC::System::Simple qw(system);
my $choice=0;
GETCHOICE: {
    print "Admin Menu\n";
    print "To add a user, press 1\n";
    print "To delete a user, press 2\n";
    print "To add a group, press 3\n";
    print "To delete a group, press 4\n";
    print "To exit this menu, press 5\n";
    print "Please choose an option: ";
    chomp($choice=<STDIN>);
    redo GETCHOICE if $choice < 1 || $choice > 5;
}
print "You chose option: $choice\n";
if($choice == 1) {
    my $nUsername;
    GETUSER: {
        print "What is new users name? ";
        chomp($nUsername=<STDIN>);
        # check that new user name is a string
        if($nUsername!~/^[A-Za-z0-9]+$/) {
            print "New username must be a string.\n";
            redo GETUSER;
        }
    }
    # check to see if user exists
    if(defined getpwnam($nUsername)) {
        print "User exists\n";
    }
    else { 
        ## FIXME delete "echo"
        system("echo","/usr/sbin/adduser","--home",
            "/home/$nUsername","--gecos",$nUsername);
        # get new user password
        my ($nPassword,$nPasstest);
        GETPASS: {
            ReadMode("noecho");
            print "Please add password: ";
            chomp($nPassword=ReadLine(0));
            print "\n";
            print "Please re-enter password: ";
            chomp($nPasstest=ReadLine(0));
            print "\n";
            ReadMode("restore");
            redo GETPASS if $nPassword ne $nPasstest;
        }
        print "Please enter group for user: ";
        chomp(my $nGroup=<STDIN>);
        # add user to group
        if(defined getgrnam($nGroup)) {
            # check to see is group exists
            print "$nGroup already exists\n";
            # add user to group
            system("echo","groupadd",$nGroup);
        }
        else { # if group does not exist create group
            system("echo","groupadd",$nGroup);
            print "$nGroup has been added.\n";
        }
    }
}
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