sysread doesn't use the buffer, so it's not limited by it. You could surely use tie to make a read use sysread.

Since 5.14, it's 8k and configurable when Perl is built.

The previous default size of a PerlIO buffer (4096 bytes) has been increased to the larger of 8192 bytes and your local BUFSIZ. Benchmarks show that doubling this decade-old default increases read and write performance by around 25% to 50% when using the default layers of perlio on top of unix. To choose a non-default size, such as to get back the old value or to obtain an even larger value, configure with:

./Configure -Accflags=-DPERLIOBUF_DEFAULT_BUFSIZ=N
[download]

where N is the desired size in bytes; it should probably be a multiple of your page size.

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