my $index = do {
    local $/;
    open my $idx, '<:raw', '/path/to/data.offsets'
        or die $!;
    <$idx>
};
open my $dat, '<', '/path/to/data.dat'
    or die $!;

my ($offset) = unpack 'i',
    substr $index, 4*rand(length($index)/4), 4;

seek $dat, $offset, 0 or warn $!;
print scalar <$dat> or warn $!;
close $dat or warn $!;
close $idx or warn;