You've composed a coherent question, congratulations, now use it in the search  [tk entry limit] [ddg://site:perlmonks.org tk entry limit] [google://site:perlmonks.org tk entry limit] tk entry limit site:perlmonks.org tk entry limit site:perlmonks.org tk entry limit

And find examples Tk validateCommand Help, Few doubts in perl-tk.

And that is only part one, part two/three/four/eleven is

widget - Demonstration of Perl/Tk widgets, Entries and Spin-boxes: 3. Validated entries and password fields.

Perl documentation documentation, Searching Perl Documentation, How to Read Perldocs

perl Tk help, RFC: Learning Perl/Tk, Perl/Tk: For Beginners, Re^3: Tkx Search for dialog box for text input , Re: TclTk interface with Perl code, is it possible? , Re^2: GUI toolkit+designer for the perl newbie , Re: Easier GUI, Re: Should I use Perl/TK?, Re^2: need a popup gui stdin, Tk Tree Tutorial ( http://www.rtapo.com/tutorials/tk_tree.html ), some Tkx tutorial links, Tutorial http://theoryx5.uwinnipeg.ca/perltk/ and http://www.perl.com/pub/1999/10/perltk/, http://perltk.org/, http://web.archive.org/web/20100310202528/http://theoryx5.uwinnipeg.ca/perltk/, How to RTFM Tk Tutorials

You are not alone :) see Tk::Button: How do I trigger a callback when button is pressed (as opposed to released)?

There are plenty of answers to be had at the speed of your fingertips