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#!/usr/local/perl5.006/bin/perl -w

...
my $unpacked = hex $temp;

print "unpacked: $unpacked\n";

>pack.pl 12
var: 12
...
temp: c0
unpacked: 192
>

16^0 <= x < 16^1 (# of hex digits is odd)  bad
16^1 <= x < 16^2 (# of hex digits is even) good
16^2 <= x < 16^3 (# of hex digits is odd)  bad
16^3 <= x < 16^4 (# of hex digits is even) good
...