use strict;
use warnings;

# First generate a sample data file
open (outFile, '>', "delme.txt") or die "Can't create test file: $!";
print outFile <<DATA;
Server: win1  ID: D: Space: 6496911360
Server: ux22  ID: D: Space: 7973826560
Server: del2  ID: U: Space: 7973228544
Server: del3  ID: U: Space: 3973228544
DATA
close outFile;

#Now the solution code
local $^I = '.bak';
local @ARGV = ('delme.txt'); #Omit this if the file is provided on the command line

while (<>) {
  chomp;
  print; #Print the original line
  my ($size) = /(\d+)$/;
  
  print " <--- too small" if $size < 4_000_000_000;
  print "\n";
}

##</code><code>##

Server: win1  ID: D: Space: 6496911360
Server: ux22  ID: D: Space: 7973826560
Server: del2  ID: U: Space: 7973228544
Server: del3  ID: U: Space: 3973228544 <--- too small