...
Name=foo
Icon=bar
Categories=a;b;c
...

##</code><code>##

#!/usr/bin/perl -w
use strict;

chdir "/usr/share/applications";

my (%collect, $out);
while (our $Fn = <*.desktop>){
    chomp $Fn;
    open Fn or die "$Fn: $!\n";
    while (<Fn>){
        $collect{name} = "$1" if /^Name=(.*)$/;
        $collect{icon} = "$1" if /^Icon=(.*)$/;
        $collect{exec} = "$1" if /^Exec=(.*)$/;
        $collect{categories} = "$1" if /^Categories=(.*);?\s*$/;
    }
    $collect{categories} ||= "Misc";
    $collect{icon} ||= "-";

    for (qw/name exec categories/){
        print "Bad value for '$Fn' ($_)\n"
            unless defined $collect{$_};
    }

    my $val = qq#prog "$collect{name}" $collect{icon} $collect{exec}#;

### More code follows

##</code><code>##

# 'categories' entry is 'GNOME;Games;Action'
push $out{GNOME}{Games}{Action}, $val;
# 'GNOME;Games;Action;FPS'
push $out{GNOME}{Games}{Action}{FPS}, $val;
# ...and so on. 

##</code><code>##

    my @levels = split /;/, $collect{categories};
    for (0 .. $#levels) {
        $out->{$levels[$_]} = {} unless exists $out->{$levels[$_]};
        $out->{$levels[$_]} = $val if $_ == $#levels;
    }
    close Fn;
}

use Data::Dumper;
print Dumper($out);