Not so. Unlike read (which waits for all 4096 characters to be read in) and <FILE> (which waits for a whole line to be read in), sysread returns as soon as possible. For example,

perl -e "$|=1; print 'test1'; sleep 1; print 'test2'" | perl -e "sysre
+ad(STDIN, $buf='', 1000); print(qq{[$buf]})"
[download]

outputs

[test1]
[download]

If what you claimed is true, it would output [test1test2].

( And I'm working on the caveat. And I've addressed the caveat in an update. )