The integer formats s, S , i , I , l , L , j , and J are inherently non-portable between processors and operating systems because they obey the native byteorder and endianness.
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If you want portable packed integers you can either use the formats n , N , v , and V , or you can use the > and < modifiers. These modifiers are only available as of perl 5.9.2. See also perlport.