Your strings having a leading character, ascii 28 (also known as the field separator). I can reproduce your error messages and results as follows:

$n1 = "\x1c-933"; ## hex( 1c ) == 28 == ^\ == Field separator
$n2 = "\x1c-1093"; 

print $n1 - $n2;;
Argument "^\-1093" isn't numeric in subtraction (-) at (eval 6) line 1
Argument "^\-933" isn't numeric in subtraction (-) at (eval 6) line 1
0
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That doesn't explain how those characters are getting into your data, but it will give you something to look for. You could try dumping the string prior to spliting using unpack:

print unpack 'C*', $the_string;
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Is this data being read from a file or a device?

Is there any chance that the data is getting the unicode bit set?