use strict;
use warnings;

my %fillers = (
    A => "Whatever is required for the A gap here",
    B => "Whatever is required for the B gap here",
    C => "Whatever is required for the C gap here",
    );

while (<DATA>) {
    s/_*(\w)_+/$fillers{$1}/eg;
    print;
}

__DATA__
function start()
    {
      __A________________________________________________;
    }

    function validate( event )
    {
      if ( B_____{
         event.preventDefault();
         C__________________;
         alert( "Wrong format for age!" );
      }
    }

##</code><code>##

function start()
    {
      Whatever is required for the A gap here;
    }

    function validate( event )
    {
      if ( Whatever is required for the B gap here{
         event.preventDefault();
         Whatever is required for the C gap here;
         alert( "Wrong format for age!" );
      }
    }