#!/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 option,  
# 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 list 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. The  
# 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();