Idiomatically,

my $btn = param ('btn');
if (defined ($btn) && $btn eq 'btn') {
    ...
}
[download]

or

use 5.010;
my $btn = param ('btn') // '';
if ($btn eq 'btn') {
    ...
}
[download]

Or see warnings for taking off the seatbelts - but don't even think about doing that until you understand what you're doing.

thanks

<button type="submit" name="btn" value="btn">Do something</button>
[download]

Excuse me, but I have some questions. First of all, why do you request the value of "btn" here? The object that you create here is a button with the handle name ("btn") and face value "btn." In other words, it creates a button that says "btn" on the face. So, if the user clicks the button that has "btn" written on it, then something happens. Okay, but why would you want to read the value of the button itself? Usually, people want to read a text value that is provided in a form such as:

<INPUT TYPE="TEXT" NAME="EmployeeAddress" VALUE="">
[download]

Or if you want the value to be hidden from view, then you would do this:

<INPUT TYPE="HIDDEN" NAME="EmployeeID" VALUE="376192">
[download]

Very rarely we would have a situation where we want to read the face value of a button.

Anyway, my second question is why would this "btn" value be undefined when you clearly give it a value. The value of "btn" object is "btn" So, this should never be undefined. I don't understand why you are getting this error or warning in the first place.

Most probably because the HTML calling the CGI has no btn ...

Don't ask me if this is good design or if this OP even cares. 🤷🏻‍♂️

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

Dude, look at this users posts, don't expect a sensible response.