ImageMagick doesn't store error messages in $! (but you knew that).

#!/usr/bin/perl

use warnings;
use strict;

use strict;
use warnings;

use Image::Magick;

my $label = Image::Magick->new(
    size => "600x600",
);

$label->Read("xc:white");
$label->Draw(
    primitive => 'line',
    points    => "300,100 300,500",
    stroke    => '#600',
);

$label->Draw(
    primitive => 'line',
    points    => "100,300 500,300",
    stroke    => '#600',
);

$label->Draw(
    primitive => 'rectangle',
    points    => "100,100 500,500",
    fill      => 'none',
    stroke    => '#600',
);

my $x = $label->Annotate(
    text      => "North West",
#    'x'       => 150,
#    'y'       => 150,
    pointsize => 40,
    fill      => 'green',
#    font      => 'Flubber.ttf', 
    font      => $ENV{WINDIR}.'\\Fonts\\Flubber.ttf',
);

warn "$x" if "$x";

$x = $label->Blur(sigma =>1);

warn "$x" if "$x";

$x = $label->Write(__FILE__.".png");

warn "$x" if "$x";

__END__
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update: Remember me saying how you need to specify x,y? See how it's comment it out? Think about it for a second.