#!/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;

print $content;