In my limited understanding of this issue, I'd say you need to *read* the data from STDIN first to get the size of the data. Like this:

#!/usr/bin/perl -w
use strict;

# must use binmode on Windows to get actual byte length of file
# otherwise line end processing will ruin our count

binmode STDIN;
local $/ = undef;
my $input = <>;
print length($input);
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With this scriptlet saved as try.pl and using some html as input:

D:\tmp>perl -0777 -pe "42" try.html | perl try.pl
5409
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I think your result from stat() is just applying to the contents of the console buffer you read from, but I am not sure about this, as I never had the idea of using stat() on STDIN, and have no idea currently what this really means.

Update: Zaxo and I performed some experimenting with this on our Linux boxes. For Zaxo (on a 2.4 kernel), stat()ing STDIN always gave 0 in field 7, for me (on a 2.2.16 kernel), it gave different values. We used this code to test:

perl -pe 'BEGIN{$|=1}' /usr/dict/words | perl -lne 'print ((stat STDIN
+)[7])'|sort|uniq
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So the message seems to be: you can't rely on stat()ing STDIN.

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com