You can read perldoc Tk::callbacks, but it really dosn't explain the simple principle. When you create the callback, you want it to be evaluated "at a later time", not "when it is created". You may run into this as an hard to track down bug, where Tk programs will crash on startup, with weird error messages. It usually is a button command being evaluated as the program starts up. Here are a couple of examples to illustrate the fine points.

#!/usr/bin/perl

 my @b;
 for my $i ( 1 .. 5 ) {
     $b[$i] = $top->Button(
         -text=>$i,
         -command=> &mybuttons($i),
                    ^^^^^^^^^^^^^^^^
                        WRONG

# Stephen O. Lidie writes:                       
#   -command expects a *code reference*, that shall, at some future ti
+me, 
#   execute.  You've called for an immediate execution, which in theor
+y could 
#   return a code ref, but doesn't here. 

#   Ya'all all are skrting around a closure, which may be created in v
+arious 
#   ways, all detailed in you know what. 

#  Try: 

-command => sub {&mybuttons($i)}

#or maybe: 

-command => [\&mybuttons, $i]
[download]

or

#!/usr/bin/perl -w
use Tk;
my $xx=99;
sub a{
     my $x=$_[0];
     my $wm= new MainWindow;
     my $l=$wm->Entry(-textvariable => \$x)->pack();
     $wm->Button(-text => "Apply",
#   -command => [ \&onApply,  "$x"])->pack(); 
    -command => [ \&onApply,  \$x])->pack();
}

# Here, you specify an anonymous list as the value of the -command opt
+ion,  
# which is a legal callback syntax. The problem is that this list will
+ be  
# created when the button is created, and the first element in the lis
+t is  
# the reference to onApply(), but the second element is the *value* of
+ $x  
# at the time the list is created, which is the initial value of $x. T
+he  
# variable $x itself has nothing to do with it anymore, so changing it
+  
# will not change this anonymous list. 

# What you can do is pass a reference to $x instead: 
# -command => [\&onApply, \$x], 
# and change your onApply() sub to reflect that. 


sub onApply{
#     my $z=$_[0]; 
      my $z = ${$_[0]};

     print "XX=$z\n";
};

&a($xx);
MainLoop();
[download]

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