Take advantage of the fact that 1 Gb = 1024 Mb, 1 Mb = 1024 Kb and 1024 = 2 ** 10:

my $kb = 1024 * 1024; # set to 1 Gb

my $mb = $kb >> 10;
my $gb = $mb >> 10;

print "$kb kb = $mb mb = $gb gb\n";
__END__
1048576 kb = 1024 mb = 1 gb
[download]

Edit: after reading your post again, I'm afraid it doesn't really do what you want (though I think it's pretty elegant), because it will round down to the nearest integer.