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#!/usr/bin/perl

open my $out, '>:raw', '/path/to/data.offsets'
...

close $in or warn $!;
close $out or warn $!;

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