Just for the record, if you want to do this with Image::Magick (TIMTOWTDI ;-), your code could look something like this:

use strict;
use Image::Magick;

# Define $image
my $image=Image::Magick->new;

# Define the image you want to convert
my $new_image = $image->Read('image.png');

# Set grayscale
$image->Quantize(colorspace=>'gray');

# Write the new file
$new_image = $image->Write('new_image.png');
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And yes, this is well documented ...