... wish to display number of characters input to entry in label on each keystroke ...

Here's one approach using entry input validation. I'm sure there are others and better.

c:\@Work\Perl\monks>perl -wMstrict -le
"use Tk;
 use Tk::LabEntry;
 ;;
 my $MW = tkinit;
 ;;
 my $entrytext = '';
 my $le = $MW->LabEntry(
   -label          => 'chars entered so far: 0',
   -textvariable   => \$entrytext,
   -validate       => 'key',
   -invalidcommand => undef,
   );
 $le->configure(
   -validatecommand => sub { showcount($le, @_) },
   );
 $le->pack;
 ;;
 MainLoop;
 ;;
 print qq{final entry: '$entrytext'};
 ;;
 sub showcount {
   my ($widget,
       $newtext,
       ) = @_;
   ;;
   my $label = $widget->cget(-label);
   $label =~ s{ \d+ \z }{ length $newtext }xmse;
   $widget->configure(-label => $label);
   1;
   }
"
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