my $text;

{
   open(local *INPUT, '<', "data")
      or die "Unable to open input file: $!\n";

   local $/; # Read whole file at once.

   $text = <INPUT>;
}

$text =~ s/(\d+)$/
   $1 >= 4_000_000_000
      ? $1
      : "$1 <---- LOW!"
   ;
/emg;

{
   open(local *OUTPUT, '>', "data")
      or die "Unable to open output file: $!\n";

   print OUTPUT ($text;
}
[download]

or if that's the entire script, you could use

#!/usr/bin/perl -pi

s/(\d+)$/
   $1 >= 4_000_000_000
      ? $1
      : "$1 <---- LOW!"
   ;
/;
[download]