Because $in_arg is scalar my $in_arg=@_;

I would say, if I really wanted to say that,

my $in_arg=scalar @_;
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but then I would just say that @array evaluated in scalar context returns the number of elements in @array and that assignment to a scalar imposes scalar context. Or something like that...

Update: since this was downvoted, why don't you actually try?

$ perl -e 'scalar my $in_arg=@_'
Can't modify scalar in scalar assignment at -e line 1, at EOF
Execution of -e aborted due to compilation errors.
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Update2: I read Re: My first module once again and now I realize I read it uncorrectly. I use a theme in which occasionally inline code is not visually distinctive enough, and GrandFather didn't put a suitable interpunction sign, so this added to my confusion and misreading. Sorry, my apologies...