Just to give you some tips ( we all needed them). If you change your increment 0.05 to a variable, you can make a widget to set it in realtime as a textvariable, and you can have adjustable speed control. You can't change the delay of the repeater dynamically, but you can fudge around it by adjusting how far something moves during that period.

#!/usr/bin/perl
use warnings;
use strict;        
use Tk;

my $mw = MainWindow->new;  
my $x = 0; my $y = 0;
my $pi = 3.1415926;
my $counter=0; 
my $c = $mw->Canvas(-width => 500, -height => 500); 
$c ->pack;
# draw axis
$c -> createLine(50, 250, 450, 250); 
$c -> createText(10, 250, -fill => 'blue', -text => 'X'); 
$c -> createLine(250, 50, 250, 450); 
$c -> createText(250, 10, -fill => 'blue', -text => 'Y'); 

$x = -(3*$pi);
my $repeater;  # declare separately so you can 
               # stop it in it's own callback

$repeater = $mw->repeat(50,sub{   # 50 millisecond delay
      $x += 0.05;
      if( $x >= +(3*$pi) ){ $repeater->cancel }  

      $y = sin($x);
      $c -> createText( $x*30+250, $y*30+250, 
               -fill => 'red', -text => '.');
     });


MainLoop;
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