Your second example also has a glitch due to precedence.  The expression:

$mask >>= 1? $_: ();
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was intended, I believe, to be:

($mask >>= 1)? $_: ();
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but is actually equivalent to:

$mask >>= (1? $_: ());
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This evaluates to true each time (1?) thus using each word ($_) as the value by which to shift $mask, and each of the strings evaluates to numerical zero.  That's why $mask never changes, and gives you a 43 (0x2b) resulting from the map applied to each value from @ARGV;