If user tries to enter 11th digit a pop up error message should be generated ... Can I do this in Perl?

As Perl runs on the back end, you'd have to send a request to the server for every key stroke, otherwise how would the back end be supposed to know what the user has typed so far?   So, in short, I'd say: No, this can't practically be done with Perl¹. (Use JavaScript instead.)

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¹ Update: ...with Perl alone plus standard HTML forms, that is. Of course, you could use Ajax. (But think twice before you decide to do so - depending on client-server round trip times, it might not be the best idea to call a server-side function for every key stroke.  As you'll need client-side Javascript (or VBScript, etc.) anyway for the Ajax glue code, it's almost certainly better to implement such a simple task as checking the length of an input string, in that client-side scripting language right away...)