use Types::Common qw( Int );

tie my $x, Int;

$x  = 5;
$x += 1;
print $x, "\n";   # prints "6"

$x = "7";         # this is fine, even though it's quoted

$x += 0.5;        # dies because 7.5 isn't an integer
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I have to guess...

You could use Tie::Scalar to make sure that a var $x will always stay an integer, by declaring STORE. But that will slow down every operation on $x.

And you can define an attribute :Int for that° as syntactic sugar.

my $x :Int = VALUE

Compare https://perldoc.perl.org/Attribute::Handlers#Attributes-as-tie-interfaces

Update

°) that Tie::Scalar class you define

my $x :Int = VALUE is exactly the sugar I want. I use

my ($text, $page) = split "\t", $line;
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in my addpdfbookmark.pl tool, $line is got from ocr, set '9O' to '90', I was very confused when

warn $text, ' on page ', $page, ' must be wrong' if $page < $lastpage;
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was triggered, I have to write a valid subroutine,

sub valid($){
    my ($text, $page) = split "\t", shift;
    $page =~ /^\d+$/ or die 'page not numbers';
    warn $text, ' on page ', $page, ' must be wrong' if $page < $lastp
+age;
    $lastpage = $page;
    return ($text, $page);
}
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, It's better to declare it as an integer, But I got 'Invalid SCALAR attribute' error, use what can make it work?

So your OCR is confusing O and 0 ?

Typing to integer won't fix that, you will need better OCR or hope you can cover the problems with heuristics.

> Invalid SCALAR attribute' error,

Because you need to implement it first, following the links I gave.

Better don't

That was an XY problem and implementing this would be a total overkill. :)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery}

It probably does not matter if the O/0 error is in the original .pdf file or if the OCR misreads it. The tr operator (refer tr/SEARCHLIST/REPLACEMENTLIST/cdsr) directly corrects the problem. Your test for a reasonable value is probably a good idea, but it really has nothing to do with the O/0 problem.

my ($text, $page) = split "\t", $line;
$page =~ tr/O/0/ and warn "Replaced 'O' with '0' in page $page";
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Bill

someone might disagree, it's no good for perl.

Perl is, contrary to C, a dynamically typed language.

A static typing, i.e. at compilation time, is nearly impossible.

Otherwise you'd need to type everything, including the result of functions. Even then....¹

A check at runtime is costly, that's why dynamically typed languages are slower, but easier to code.

As I already said, you could tie the variable with a self-made STORE (This, Value) function which does your checks on Value before storing it.

Attribute::Handlers has an example for Syntactic sugar to "hide" the tie-ing procedure behind an attribute.

I'm not aware of any CPAN modules already providing that, especially because there are certainly different models how to react on wrong types ( die , warn , transform, combine)

I already gave you all the links, you'd need to do it yourself for your own needs.

For instance 1E0 is a whole number in float notation, opinions will differ.

I'm saying all of this for completeness, because it's a total overkill for your problem.

There is no magic wand which can do the heavy lifting for you...

Cheers Rolf
_{(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery}

¹) Perl, like other dynamic languages, does a lot of DWIM typecasting of variables. This doesn't translate well if you want to keep it fast.