$str1 = "But I think the device doesn't allow writes or something and that's causing this issue.";
$str2 = "But I think the device allow writes or something and that's causing this issue.";

##</code><code>##

open (FHH, ">error.txt")|| die $!;
$str1 = "But I think the device doesn't allow writes or something and that's causing this issue.";
$str2 = "But I think the device allow writes or something and that's causing this issue.";
my $line = 0;
my $pat = qr/$str2/;
while($str1){
   $line++;
   if($str1 =~m/($pat)/igs){
      my $pre = $`;
   }
   else{
     print FHH "Line $line:$pre \'$pat\' is missing from $str2."
   }
     
}