It just don't work as I thought. 
use strict;

# generate a 64-bit all-1 mask
my $mask;
for (my $i=0; $i<64; $i++) {
    vec($mask,$i,1)=1;
}

# get a byte of value from 17
my $value = vec($mask,16,8);
print "$value\n";

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In my opinion, $value should be string "255". Actually, 
vec($mask,0,8)

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give me that, but with start index other than 0, it always give me "0". So how does it work?

In reply to how does vec() return? by llancet

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