It is printing 11 because you have the:

my $in_arg=@_;
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Which is giving scalar context to the assignment from @_ which puts the length of the array in $in_arg you want to do either:

my ( $in_arg ) = @_;
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or

my $in_arg = shift;
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There is a difference between the two related to the side-effects but you won't notice it in your code.

/J\