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
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).
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