I ran the code you post and throw an error:

No data given to hexdump. at C:/Perl/site/lib/Data/Hexdumper.pm line 2
+88.
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I've never used Compress::Raw::Zlib. but I think compress lib doesn't know what string or number is, it just treat all as binary data. So do you try to put number in to a file, binmode it and test it again? or just:

$num_data = 7;
open(my $fh, '<', \$num_data);
binmode($fh);
.......
.......
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HIH.