From the IO::Handle documentation:

# setvbuf is not available by default on Perls 5.8.0 and later.
use IO::Handle '_IOLBF';
$io->setvbuf($buffer_var, _IOLBF, 1024);
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However, the PerlIO::buffersize module adds a PerlIO layer that can set the buffersize when opening a file:

open my $fh, '<:buffersize(65536)', $filename;