I don't know whether it's possible to create custom events with any event structure you'd like to define, but I fear it's not.

I'm not sure whether an event can be cancelled, but you can definitely not schedule the following one, and do nothing in the current one.

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

use Time::HiRes qw{ usleep };
use Tk;

my $mw = 'MainWindow'->new(-title => 'Event demo');
$mw->bind('<<RowDone>>' => \&next_row);

my $drawarea = $mw->Frame->pack(-side => 'top', -fill => 'both');
my $p = $mw->Photo(-width => 500, -height => 500);

my $canvas = $drawarea->Canvas(
    -relief      => 'ridge',
    -width       => 500,
    -height      => 500,
    -borderwidth => 4
)->pack;

$canvas->createImage(0, 0, -anchor => 'nw', image => $p);

$mw->Button(-text => 'Plot', -command => \&init)
    ->pack(-side => 'left');

$mw->Button(-text => 'Stop', -command => \&stop)
    ->pack(-side => 'left');

$mw->Button(-text => 'Quit', -command => sub { Tk::exit() })
    ->pack(-side => 'left');

MainLoop();


{   my ($y, $done);

    sub init {
        undef $done;
        $y = 0 if ! defined $y || $y == 500;
        $mw->eventGenerate('<<RowDone>>');
    }

    sub next_row {
        return if $done;
        for my $x ( 0..499 ) {
            my $quality = long_computation($x, $y);
            $p->put(color($quality), '-to', $x, $y);
        }
        $canvas->update;
        if (++$y < 500) {
            $mw->eventGenerate('<<RowDone>>', -when => 'tail' );
        }
    }

    sub stop { $done = 1 }
}

sub color { qw( red blue )[ rand 2 ] }

sub long_computation { usleep(100) }
[download]

*Update*: return from next_row at the very beginning.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]