-command => \&sub_opt,

##</code><code>##

-command => sub { \&sub_opt },

##</code><code>##

#!/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.