What's happening here is that when you do:

my @array = map { s/^-// } @ARGV;
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two things are happening. First, the subtitution is changing $_ which is actually a reference to an element in @ARGV. Like in foreach when you change $_ in a map or grep it affects the item in the array being based to map or grep. As for the values in @array, they are always 1 in your example because the result of s/^-// is 1 (true) when the substitution succeeds, and false otherwise.