use warnings; and in this case specifically, use strict; will point you to your errors. Correct those and you'll be much further along.

You're not accessing the elements of the hash reference correctly (perl thinks you're trying to access a hash, not a reference to one). Use the -> dereference operator between the reference and the first key to distinguish it as a ref, eg: %{$professionsXML->{$key}}.

Also, there are values in that structure that are not hashrefs, so you need to explicitly check for this. Here's (a messy) working example to show you what I mean by my statements. I most likely wouldn't have written it like this, but I wanted to point out where the mistakes are within the code structure you provided

use warnings;
use strict;

use XML::Simple;
use Data::Dumper;

my $xmlSimple = new XML::Simple(KeepRoot   => 1);
my $professionsXML = $xmlSimple->XMLin("in.xml");
print Dumper($professionsXML);

print qq(0.1: $professionsXML->{'profs'}{'name'}\n);
print qq(0.2: $professionsXML->{'profs'}{'profcats'}{'name'}\n);

foreach my $key (keys %{$professionsXML}) {
    next if ref $professionsXML->{$key} ne 'HASH';
    print qq(1: $key\n);
    foreach my $key2 (keys %{$professionsXML->{$key}}) {
        next if ref $professionsXML->{$key}{$key2} ne 'HASH';
        print qq(2: $key2\n);
        foreach my $key3 (keys %{$professionsXML->{$key}{$key2}}) {
            next if ref $professionsXML->{$key}{$key2}{$key3} ne 'HASH
+';
            print qq(3: $key3\n);
            foreach my $key4 (keys %{$professionsXML->{$key}{$key2}{$k
+ey3}}) {
                print qq(4: $key4\n);
            }
        }
    }
}

exit(0);
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