sub print
{
    print $value unless $value eq '-';
}
[download]

...

Incidentally, why does this  print sub [not] simply call itself recursively??

Its a namespace problem (feature? artifact?). Your new  print is defined in  main and the built-in  print is defined in  CORE and the  CORE namespace has search precedence. Try the following:

>perl -wMstrict -le
"sub print { CORE::print('foo') }
 ::print;
"
foo

>perl -wMstrict -le
"sub print { ::print('foo') }
 ::print;
"
Deep recursion on subroutine "main::print" at -e line 1.
Terminating on signal SIGINT(2)
[download]

Thanks, AnomalousMonk, for the explanation. Your statement:

the CORE namespace has search precedence.

raises a new question: How, then, does the original code ever find the print sub defined in namespace main if the sub isn’t explicitly qualified with its namespace (i.e., main::print)? Why doesn’t Perl always find CORE::print?

I think I found the answer: the original code works only because it uses a reference to the sub:

-command=> \&print
[download]

and in this case the reference is disambiguated using lexical scope. I found an explanation of the latter in the Camel Book, 4th Edition, “Name Lookups,” pages 62–65. Is there any documentation on how Perl looks up subroutine names when a subroutine is actually called (as opposed to being referenced)?

Thanks,

Athanasius <°(((>< contra mundum

this code ... is allowing inputs other than numbers

Not so! For the record, here is the exact code I am running (using Strawberry perl v5.16.0 built for MSWin32-x86-multi-thread-64int, running under Windows Vista 32-bit, and with Tk 804.030):

#! perl
use strict;
use warnings;
use Tk;

my $value;
my $mw  = new MainWindow;
my $ent = $mw->Entry(-textvariable    => \$value,
                     -validate        => 'key',
                     -validatecommand => sub { $_[0] =~ /^(?:|-|\d+|-\
+d+)$/ },
                     -invalidcommand  => \&lam_num_error)->pack();

my $print_button = $mw->Button(-text    => "Print",
                               -command => \&printx,
                               -font    => "ansi 10 bold")->pack();

my $reset_frm = $mw->Frame();
$reset_frm->pack(-fill => 'both');
my $reset_button = $reset_frm->Button(-text    => "Reset",
                                      -command => \&do_reset,
                                      -font    => "ansi 10 bold")->pac
+k();
MainLoop;

sub printx
{
    print $value unless $value eq '-';
}

sub do_reset
{
    $ent->delete(0, 'end');
}

sub lam_num_error
{
    $mw->messageBox(-message => "The input must be an integer.");
}

__END__
[download]

This lets me enter -42, 763, etc., but if I try to enter a letter, or a punctuation symbol, or a minus sign anywhere after the first character, or even a space, I immediately get the message “The input must be an integer.” and my input never makes it into the Entry box. In other words, the code is working exactly as required.

Athanasius <°(((>< contra mundum

I apologize Athanasius. It does do exactly what you say. Thank you very much for being so helpful! :-)