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?