See Re^3: Portable length() in bytes.. The bottom line is length will not calculate how many bytes will go out, and syswrite is expecting a character count. (Don't get upset if what you are writing is binary data; in that case you should have 8-bit characters.) See some examples (note that v255 is utf-8 encoded in perl, while "\xff" is not, and -CO tells perl that STDOUT expects utf8): 
$ perl -we'$x = v255;
 {use bytes; print STDERR "len:",($len=length $x),"\n" }
 print STDERR "wrote: ",($x = syswrite STDOUT, $x, $len),"\n"'|cat
len:2
wrote: 1
˙

$ perl -we'$x = "\xff";
 {use bytes; print STDERR "len:",($len=length $x),"\n" }
 print STDERR "wrote: ",($x = syswrite STDOUT, $x, $len),"\n"'|cat
len:1
wrote: 1
˙

$ perl -CO -we'$x="\xff";
 {use bytes;print STDERR "len:",($ln=length $x),"\n"  }
 print STDERR "wrote: ",($x = syswrite STDOUT, $x, $len),"\n"'|cat
len:1
wrote: 1
Aż

$ perl -CO -we'$x=v255;
 {use bytes; print STDERR "len:",($len=length $x),"\n" }
 print STDERR "wrote: ",($x = syswrite STDOUT, $x, $len),"\n"'|cat
len:2
wrote: 1
Aż

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The "length" passed to syswrite is useless; it expects and returns character length and offset. And whether the string being output is 1 byte or 2 bytes, it's just one character, and will be output as either 1 or 2 bytes depending on the output filehandle, not on how perl has it encoded.

In reply to Re^5: Portable length() in bytes. by ysth
in thread Portable length() in bytes. by William G. Davis

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