[root@devel3 root]# cat reader.pl
#!/usr/bin/perl
open $fh, '/root/big.file' or die $!;
1 while read( $fh, $buf, $ENV{BLOCK_SIZE} );
close $fh;

[root@devel3 root]# ll big.file
-rw-r--r--    1 root     root     100000000 Dec 31 04:59 big.file

[root@devel3 root]# BLOCK_SIZE=1024 time perl /root/reader.pl
0.11user 0.08system 0:00.20elapsed 94%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+31minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=2048 time perl /root/reader.pl
0.05user 0.12system 0:00.16elapsed 104%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+33minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=4192 time perl /root/reader.pl
0.03user 0.10system 0:00.13elapsed 94%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+32minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=4096 time perl /root/reader.pl
0.05user 0.06system 0:00.11elapsed 94%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+33minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=1024 time perl /root/reader.pl
0.09user 0.11system 0:00.19elapsed 103%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+33minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=2048 time perl /root/reader.pl
0.03user 0.13system 0:00.15elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+34minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=4096 time perl /root/reader.pl
0.01user 0.11system 0:00.11elapsed 102%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+33minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=8192 time perl /root/reader.pl
0.02user 0.08system 0:00.09elapsed 107%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+33minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=16384 time perl /root/reader.pl
0.01user 0.07system 0:00.08elapsed 98%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+34minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=65536 time perl /root/reader.pl
0.00user 0.07system 0:00.07elapsed 93%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+45minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=262144 time perl /root/reader.pl
0.02user 0.12system 0:00.13elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+95minor)pagefaults 0swaps
[root@devel3 root]# BLOCK_SIZE=524288 time perl /root/reader.pl
0.01user 0.18system 0:00.25elapsed 74%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (275major+159minor)pagefaults 0swaps
[root@devel3 root]#