The short answer is yes. A hash key can be any string as long as that string is unique. The "classic example" is printing a file and omitting duplicate lines that have been seen before....

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

my %seen;
while (<DATA>)
{
   print unless $seen{$_}++;
}

#Prints:
#1 2 this is a line with 1 and 2
#3   a line with just 3
#5,6 five before 6
#5,6 five before six (different)
#6,5
#blah
__DATA__
1 2 this is a line with 1 and 2
3   a line with just 3
1 2 this is a line with 1 and 2
3   a line with just 3
5,6 five before 6
5,6 five before six (different)
6,5
blah
blah
[download]

In general, I would not concatenate keys together. Well, until you have learned multi-dimensional structs, I would say that concatenating 2 things and not more is "ok".

There are some advanced techniques where say 8 dimensions can be combined into a single hash key, but those situations are seldom and not applicable for normal code (a way to describe state tables and a way to simplify horrifically complex logic statements).