Maybe you can explain line by line what your code is supposed to do?

  elsif($choice eq 'get')
  {
    open(FH,"+>>database.txt")|| die 'Cannot open the file';
    print FH my @database;
    my $mykeys=(\@database);
    print STDOUT @{$mykeys},"\n";
  }
[download]

Also, most likely, your course material contains all the information needed for reading and writing from a text file.

I recommend using DBD::SQLite instead if this is not homework.

I've taken the liberty of annotating the code

# -----[ Help Me Get It Right ]---------------------------------------
+--------

use strict;
use warnings;


# -----[ Modules ]----------------------------------------------------
+--------

use Data::Dumper;


# -----[ Globals ]----------------------------------------------------
+--------

my %database;
my @employee_details;
my @mykeys;


# -----[ Kill any database I might already have ]---------------------
+--------
open(FH,"+>>database.txt")|| die 'Cannot open the file';
print FH %database;
close FH;

# ----[ Perl isn't confusing enough so put subroutine in the middle of
+ main ]-
sub client
{
    # -----[ Blindly accept anything they throw at me ]---------------
+--------
    my $choice=shift @_;

    if($choice eq 'add')
    {
        # -----[ Get some data from the user ]------------------------
+--------
        print STDOUT 'Enter employee details',"\n";
        print STDOUT 'Enter employee id',"\n";
        my $employee_id=<STDIN>;
        chomp $employee_id;
        print STDOUT 'Enter employee name',"\n";
        my $employee_name=<STDIN>;
        chomp $employee_name;
        print STDOUT 'Enter employee salary',"\n";
        my $employee_salary=<STDIN>;
        chomp $employee_salary;
        print STDOUT 'Enter employee phone number',"\n";
        my $employee_ph=<STDIN>;
        chomp $employee_ph;

        # -----[ Shove well-defined data into a mysterious array ]----
+--------
        @employee_details=($employee_name, $employee_salary,$employee_
+ph,"\n");

        # -----[ Shove mysterious array into a mysteriouser hash ]----
+--------
        $database{$employee_id}=(\@employee_details);

        # -----[ Now (try to) print to a File Handle I closed in main 
+]-------
        # -----[     If I do this enough, maybe Perl won't notice     
+]-------
        print FH %database;
    }

    elsif($choice eq 'get')
    {
        # -----[ Open the file I emptied in main ]--------------------
+--------
        open(FH,"+>>database.txt")|| die 'Cannot open the file';

        # -----[ Empty it again just to be sure it's dead      ]------
+--------
        # -----[      Make a new empty array while we're at it ]------
+--------
        print FH my @database;

        # -----[ Fetch the address of the empty array I just created ]
+--------
        my $mykeys=(\@database);

        # -----[ Fancy way to print a blank line ]--------------------
+--------
        print STDOUT @{$mykeys},"\n";
    }
}

# -----[ Okay, let's continue our original train of thought ]---------
+--------
# -----[     We could have given the user a clue here about what to en
+ter  ]--
# -----[     But that would take the fun out of it                    
+     ]--
# -----[     We could give no prompt at all, but they might just sit t
+here ]--
# -----[     Let's at least throw them a bone so they know they are ch
+oking]--
# ----=[     Besides, only trained people will use this so do not expl
+ain  ]--
print STDOUT "Enter your option\n";

# -----[ Get the user "option" ]--------------------------------------
+--------
my $input=<STDIN>;
chomp  $input;

# -----[ Now let's go back and call the subroutine with whatever we go
+t ]-----
client($input);

# -----[ Disappear without ceremony ]---------------------------------
+--------
[download]

So, let's see what it does.

D:\PerlMonks>emp1.pl
Enter your option
add
Enter employee details
Enter employee id
1
Enter employee name
Mike
Enter employee salary
15
Enter employee phone number
123456789
print() on closed filehandle FH at D:\PerlMonks\emp1.pl line 55, <STDI
+N> line 5.

D:\PerlMonks>
[download]

Darn, Perl caught me. Again. I wonder how long it will take before it slips by?

Sorry for the terrible code i have written..As i am new to perl, with the basic knowledge what i have ,tried to write a code to store the details of an employee into a text file as hash(%),so that i can store the id as a key and all details in a list assigned to the key.so i can access the details with the key when ever i need.. This is what i need..Can u please help me with that..

sorry for that.. i reopened the file as i can't the content in the file handle.I hope it is not necessary there.

Which line of the code does your explanation refer to? What about the other lines? Why did you write them if you don't explain them?

Does your database.txt file contain any data? Your 'add' logic tries to store it, but the filehandle is closed so nothing can be written. Your 'get' logic opens the file but never reads from it. Even if there was some data in there, you would never see it.

UPDATE: see File IO Tutorial.

# ...
  my %database;
  my @employee_details;
  my @mykeys;
  open(FH,"+>>database.txt")|| die 'Cannot open the file';
  print FH %database;
  close FH;
[download]

Similarly, the elsif block ("get") does not make more sense.

Also I would strongly advise you to indent your code properly. This is very important for understanding the logic of a program.