I think the problem is just in @in and not the pack/unpack .. When you pack @in in H2's, it's ('0','1','0','2','0','3') ... I hardcoded it as ('01', '02', '03') in a test run and it gave your desired output..

Probably not the best way, but this works:

my @in = grep length $_, split(/(..)/, $string);
shift @in;  # just 1 shift to pull off '0x'
my $bytes = scalar(@in);  # no longer need the div by 2
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