Per sub routine

I am working on a perl script that would print a value based on a menu option. Below is the menu option.
    Select one of:
    1. Print the Current Value
    2. Add a number to current value
    3. Subtract a number from the current value
    4. Multiply the current value by a number
    5. Divide the current value by a number
    6. Raise the current value to a power
    7. clear the current value to 0
    8. Exit";

the problem i am having is with the loop. I want it loop until the user enter 8 to exit.

#!/usr/bin/perl
    #use strict;
    use warnings;





    # Subroutine prototypes
    sub do_exit();
    sub current_value();
    sub add_number();
    sub subtract_number();
    sub multiply_number();
    sub divide_number();
    sub raise_number();
    sub clear_number();
$value=0;


    # Print the menu selection
    print "
    Select one of:
    1. Print the Current Value
    2. Add a number to current value
    3. Subtract a number from the current value
    4. Multiply the current value by a number
    5. Divide the current value by a number
    6. Raise the current value to a power
    7. clear the current value to 0
    8. Exit";
print "\n";


    # Get the user's input
 $menu_item = <>;
while ($menu_item ne 4) {
    chomp($menu_item);
    # Define the actions to take
    if ($menu_item == 1) 
	{
        current_value();           
   	 } 
	elsif ($menu_item == 2) 
	{
        add_number();
	}
	elsif ($menu_item == 3) 
	{
        subtract_number(); 
	  }
     	elsif ($menu_item == 4)
	 {
       multiply_number(); 
	}
  	 elsif ($menu_item == 5) 
	{
       divide_number(); 
	}
   	elsif ($menu_item == 6) 
	{
       raise_number(); 
	}
 	elsif ($menu_item == 7) 
	{
        clear_number(); 
	}
	 elsif ($menu_item == 8) {
         do_exit();
	}

     else {    
        print "I didn't understand the command.\n";
        do_exit();
    }

    exit 0;

} 
   

    #-------------------------------------------------------------
    sub current_value() {
		
	print $value;    
	print "\n";
	return;
    }

    sub add_number() {
	print "enter number to add,\n";
	$number = <STDIN>;
 	$value = ($number +$value);
 	print $value,"\n";
        return;
    }
    
     sub subtract_number() {
	print "enter number to subtract,\n";
	$number = <STDIN>;
 	$value = ($value-$number);
 	print $value,"\n";
        return;
    }

	  sub multiply_number() {
	print "enter number to multiply,\n";
	$number = <STDIN>;
 	$value = ($value*$number);
 	print $value,"\n";
        return;
    }

	sub divide_number() {
	print "enter number to divide,\n";
	$number = <STDIN>;
	if ($number ==0){
	print "please choose number greater than 0, \n";
}
 	else {
	$value = ($value/$number);
 	print $value,"\n";
}
        return;
    }
	  sub raise_number() {
	print "enter number to multiply,\n";
	$number = <STDIN>;
 	$value = ($value**$number);
 	print $value,"\n";
        return;
    }
	  sub clear_number() {
 	$value = (0);
 	print $value,"\n";
        return;
    }
    sub do_exit() {
        print "Exiting...\n";
        return;
    }

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