Using a subroutine and a hash will help take out some troubles.
Something in this like:

use warnings;
use strict;

my $choice;

my $user_grp = read_doc("/etc/passwd");

my %admin_menu = (
    1 => sub {
        my $username = get_input("Enter username: ");
        if ( exists $user_grp->{$username} ) {
            print "\nUser ", $username, " exist!";
            return;
        }

        system( "useradd", "-u $$", "-g", "users", "$username" );
    },
    2 => sub {
        my $username = get_input("Enter username: ");
        if ( exists $user_grp->{$username} ) {

            system( "userdel", "$username" )
              if get_input("Do you really want to remove the $username
+?: ") =~
              /\by\b/i;
        }
    },

    3 => sub {    # do for groupadd
    },
    4 => sub {    # do for groupdel

    },
    5 => sub { get_input("Exiting the Program\n"); exit(); },
);

do {
    admin_menu();
    $choice = get_input("Enter your choice: ");
    if ( $choice < 1 or $choice > 5 ) {
        admin_menu();
    }
    else { $admin_menu{$choice}->(); }

} while (1);

sub get_input {
    my $prompt = shift;
    print $prompt;

    chomp( my $value = <> );
    return $value;
}

sub read_doc {
    my $file = shift;
    my %user;
    open my $fh, '<', $file or die $!;
    while (<$fh>) {
        my ( $name, $grp ) = ( split /:/ )[ 0, 4 ];
        $user{$name} = $grp;
    }

    return \%user;
}

sub admin_menu {
    print << "UNIX_ADMIN";

Admin Menu

To Add a User,     press 1:
To Delete a User,  press 2:
To Add a group,    press 3:
To Delete a group, press 4:
To Exit this Menu, press 5:

UNIX_ADMIN

}
[download]