It will return whatever bytes are available. 
$ perl -e'$|=1; print "a"; print "b"; sleep 1; print "c"' | \
    perl -le'print "[$buf]" while sysread(STDIN, $buf="", 4096);'
[ab]
[c]

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In reply to Re^5: IO::Handle read error description? by ikegami
in thread IO::Handle read error description? by Marcello

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