Are you sure you are quoting Learning Perl fairly? I suspect that what you saw was

my ($blah) = @_;
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which, like

my $blah = shift;
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assigns the first element of @_ to $blah, although the form using shift has the additional effect of removing this first element from @_. Or maybe you saw

my @blah = @_;
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which assigns all of @_ to the array @blah.

In contrast, what you quoted,

my $blah = @_;
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just assigns to $blah the size of @_.