use strict;
use warnings;
use GD;

my $in_file = qw[foo.png];
my $ni_file = $in_file.".ni.png";
my $image = GD::Image->new($in_file);

my $i = 0;
my $t = $image->colorsTotal();

while($i < $t) {
    my( @c ) = $image->rgb( $i );
    my $g = ( $c[0] + $c[1] + $c[2] ) / 3; # Color::Calc::grey

    $image->colorDeallocate($i);
    $image->colorAllocate( $g, $g, $g );
    $i++;
}

write_file( $ni_file, $image->png );
sub write_file {
    my $i = shift;
    open DISPLAY, ">$i" or die "can't clobber $i $!";
    binmode DISPLAY;
    print DISPLAY @_;
    close DISPLAY;
}