$mac = join( ':' => ($mac =~ /(..)/g) );
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Why not drop the parens?

$mac = join( ':' => ($mac =~ /../g) );
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BTW, I did something a bit similar today to get all the pod sections with name $sname in file $fname as a string:

join("", my_slurp_file_function($fname) =~
   /^=begin[ \t]+$sname[ \t]*\n(.*?)^=end[ \t]+$sname[ \t]*$/msg);
[download]