#!/usr/bin/perl

open(DATA,"<a.txt") or die "Caanot read $!";
@info=<DATA>;
close(DATA);

open(FH,"+>b.txt") or die "Not able to create";
foreach (@info) {
    $_ =~ s/NULL//g;
    $_ =~ s/ //g;
}
print FH "@info";
close(FH);
system("mv b.txt a.txt");

a.txt data
=====
This is    a test line
Contains NULL values NULL
Testing   Testing   in Progress