Take a look at that stack overflow reply that shows how the bitmask is constructed. This bitmask breakdown should be almost a blueprint to your code. I also tend to struggle with pack so using the left shift operator for each group of bits:

my ($sec, $min, $hr, $day, $mth, $yr) = localtime($info[10]);
$mth++;
$yr -= 80; # change offset from 1900 to 1980
{ use integer; $sec /= 2; } # convert sec units from 1 to 2

my $packed = ($year << 25) + ($mth << 22) + ($day << 16)
       + ($hr << 11) + ($min << 5) + $sec;
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