You can catch mouse events from your window and then do whatever you want in the assigned subs:

use strict;
use warnings;

use Win32::GUI;

my $draw  = 0;

my $mw = Win32::GUI::Window->new(
    -title => "Click anywhere to draw",
    -pos   => [100,100],
    -size  => [500,500],
    -onMouseDown => \&down,
    -onMouseUp   => \&up,
    -onMouseMove => \&move,
);

my $DC = $mw->GetDC;
$mw->Show();
Win32::GUI::Dialog();

exit(0);

sub down {
    my ($object, $x, $y) = @_;
    $draw = 1;
    $DC->TextOut(10,10,"start $x,$y");
    return 1;
}

sub up {
    my ($object, $x, $y) = @_;
    $draw = 0;
    $DC->TextOut(400,10,"end $x,$y");
    return 1;
}

sub move {
    my ($object, $x, $y) = @_;
    return unless $draw;
    $DC->SetPixel($x,$y,[192,192,192]);
    $DC->TextOut(200,10,"at $x,$y");
    return 1;
}
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