Your screen capture doesn't work. Based on this, here's a sub that does a full capture.

#!/usr/bin/perl

use strict;
use warnings;
use Tk;
use Tk::WinPhoto;

my $mw = MainWindow->new();
full_capture();

MainLoop;

sub full_capture {
    my @id = grep { $_ =~ 'Window id' } split( "\n", `xwininfo -root` 
+);
    my @ids = split( ' ', $id[0] );
    ( my $id ) = grep { $_ =~ /0x/ } @ids;

    my $image = $mw->Photo(
        -format => 'Window',
        -data   => oct( $mw->id ),
        -data   => oct('0x00022'),
        -data   => oct($id)
    );

    my $pathname = './rootwindow.' . time . '.png';
    $image->write( $pathname, -format => 'BMP | PPM | XPM' );
}
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