Perl's buffered IO (read, readline) always do 4k reads, repeating if necessary. sysread, on the other hand, simply passes the requested number of bytes to the OS.

$ 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 'rea
+d(0'
read(0, ""..., 12345)                   = 0
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