#!/usr/bin/perl

use strict;
use warnings;

my $file = './test.txt';
my $content;

{
  local $/;
  open(my $fh,'<',$file) or die $!;
  $content = <$fh>;
  close $fh;
}

$content =~ s/#String:.*?sa\d{3}s\d{2}\n?//sg;

open(my $fh_w,'>',$file) or die $!;
print $fh_w $content;
close $fh_w or die $!;