Yeah, that square-bracket suroutine syntax is one of the trickiest aspects of Tk. Here are a couple of other examples which demonstrate things to watch out for(gleamed from usenet, probably written by Ala Qumsieh, a Tk guru). The details are in "perldoc Tk::callbacks" , but that is hard to get without examples.

#!/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();
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AND##########################################

#!/usr/bin/perl

#for example the following is wrong 
my $row = 0;
my $column = 0;

for (my $i = 9; $i >= 0; $i--)
{
$button{$i} = $mw->Button(-text    => "$i",
                         -width   => '3',
                         -height  => '1',
                         -command => &numpress($i))
                  ->grid(-row     => $row,
                         -column  => $column);

$column++;
if($column > 2){$column = 0; $row++;}
}
MainLoop;

####################################################### 
# Now the problem is here: 

  -command => &numpress($i))

# The thing being assigned to the "-command" attribute needs  
# to be an anonymous subroutine, a reference to a named subroutine,  
# or else a reference to an array whose elements are:  
# named_subroutine_ref, arg1(, arg2 ...) -- in other words,  
# either of the following would be the right way to do what you want: 

  -command => sub { numpress( $i ) }

# or 

  -command => [ \&numpress, $i ]

#The way you had it written, your subroutine is actually being called 
+ 
#when the Button is being created, and Perl/Tk is trying to use the  
#return value of the sub as the value for "-command" -- not good.
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