use strict;

# open in read/write mode
open(DATA, "+< a.txt") or die "Cannot read $!";
my @info = <DATA>;
foreach (@info) {
  $_ =~ s/NULL//g;
  $_ =~ s/ //g;
}

seek(DATA, 0, 0); # rewind back to beginning

foreach(@info) {
  print DATA;
}

truncate(DATA, tell(DATA));
close(DATA);