$ strace perl -e'read STDIN, $_, 12345' </dev/null 2>&1 | grep 'read(0'
read(0, ""..., 4096)                    = 0

$ strace perl -e'sysread STDIN, $_, 12345' </dev/null 2>&1 | grep 'read(0'
read(0, ""..., 12345)                   = 0