The error is correct, because this expression: $#{@$head} does not do what you think it should. I assume you want the number of elements in @$head there. If head were an array, your code would look like this:

sub new {
    ...
    my @head = (1,2,3);
    my $cols = $#head;
    ...
};
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This already is wrong, because $#head is the index of the last element, so $cols will be 2, not 3. But to translate the code above, just follow References Quick Reference and write:

sub new {
    ...
    my $head = [1,2,3];
    my $cols = $#{$head};
    ...
};
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