Hi Guys i just made some modifications that you suggested me and here is the code again,Please any advices or improvements are more than welcomed.

#!/usr/bin/perl -w

use strict;

my $ID;
my $userInput;

open TEMPFH,"DB.txt" or die "Sorry Cant Open File...$!";
#reading the File For Getting the Last id To Support Auto increment.Th
+ere is another way to do it later
#using Tail .
while(<TEMPFH>){
$_ =~/^(\d*).*$/;
    $ID=$1 if eof;
}
close TEMPFH;


print "Welcome To My Simple Phone Book:)\n";
print "What Do You want me To Do\n";
while(1){
print "[L]ist,[A]dd,[D]elete,[Q]uit\n";

chomp($userInput = <STDIN>);

 if (uc $userInput eq "L" or uc $userInput eq "LIST"){
    &list($ID);

   }

 elsif (uc $userInput eq "A" or uc $userInput eq "ADD"){
    &add($ID);
 
   }

    elsif(uc $userInput eq "Q" or uc $userInput eq "QUIT"){
        print "Thank You,Keep Visiting :D\n";    
    last;
    }
    
    elsif(uc $userInput eq "D" or uc $userInput eq "DELETE"){
        &delete($ID);
        

        }
else{
print "Sorry Invalid Option...Try Again";
}
print "\n";
}

######################################################################
+#################################################
sub add{

 open WRITEFH,">>DB.txt" or die "Sorry Cant open The Database..$!";

my $name;
my $number;
        while(1){
    print "Please enter the name\n";
    print "Please Note that Only chars,numbers and underScore are aLlo
+wed with max 30 char and min One Char \n";
    chomp($name = <STDIN>);
    if ($name !~ /^\w{1,30}$/){
    print "Sorry Invalid Name Try Again\n";
    next;
  }
      else{
      last;


     }
}
    while(1){

    print "Please Enter The Number\n";
    print "Please Note that Only Numbers are allowed With max 30 digit
+ and min 1 digit\n";
    
    chomp($number=<STDIN>);
    if ($number !~ /^\d{1,30}$/){
    
    print "Sorry Invalid Number..Try Again\n";
    next;
}
else{
last;
}
}
    $_[0]++;
    print WRITEFH "$_[0]:$name:$number\n";
    print "Added Succefully\n";
close WRITEFH;
}
sub delete{
 my $currentID=$_[0];
 my $userToDelete;
 my $myid=1;
print "Please Enter the Name You Wana Delete\n";
        chomp($userToDelete = <STDIN>);
        open DELETEFH,"DB.txt" or die "Sorry Couldnt Open DB...$!";
        open TEMP,">temp.txt" or die "Sorry Couldnt Open DB...$!";
        
     
        while (<DELETEFH>){
    
    if (/^\d*:$userToDelete:\d*$/i)
   {
    $_[0]--;
    next;
   }

    else{
        $_ =~ /^\d*:(.*)$/i;
        print TEMP "$myid:$1\n";
        $myid++;
            }
                }
        if($_[0] < $currentID){

  print "Successfully Deleted user:$userToDelete\n";
}
else{
print "Error..Deleting User Please Check the username Again..,\n";
}
        close DELETEFH;
        close TEMP ;
        rename "temp.txt","DB.txt" ;
$myid=1;



}
sub list{
 open READFH,"DB.txt" or die "Sorry Cant Open the DataBase...$!";
    for (<READFH>){

        print;
}
 close READFH;


}
[download]

Thanks in advance.