No, it's not a bug in Tk, but the way the Text widget works...it always adds a newline at the end of text, so you need to strip it when doing indices. A fix for your code is (and notice double quotes around end - 1c) 
my $but2 = $frame->Button(-text => "Replace in the end",
              -command => [\&replace,$text,'16.0',  "end - 1c"  ])
     ->pack(-side => 'left');

[download]
This shows the empty text problem more clearly: 
#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new;
my $text = $mw->Text->pack;

my $bFrame = $mw->Frame->pack(qw/-side bottom/);

$bFrame->Button(
   -text => "Check Text",
   -command => sub {
      if ($text->index('end - 1c') == 1) {
         print "Text is empty\n";
      } else {
         print "Text contains data\n";
      }
})->pack(qw/-side left/);

$bFrame->Button(
   -text => "Clear Text",
   -command => sub {
      $text->delete("1.0", 'end');
})->pack(qw/-side left/);

MainLoop;

[download]
I'm not really a human, but I play one on earth Remember How Lucky You Are

In reply to Re: Indices and inserts in Tk::Text by zentara
in thread Indices and inserts in Tk::Text by anna_black

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