my @a = 1..10;

{
        my $s = shift @a;

        print "$s\n"

        #result: 1 which is the 1st @a array element
}

{
        my $s = @a;

        print "$s\n"
     
        #result: 9 which is the number of @a array elements
}
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The same is valid for shift and @_.
You may want to take a look at perldoc -f shift.