As the others have mentioned, what you're tripping over is a DWIM (Do What I Mean) feature of Perl: under certain circumstances, some words are assumed to have quotes around them when used on the left of the double arrow (or "fat comma") and in hash lookup curlies. (Did I miss any cases? There's also special rules for package names, but they're a bit different.) You can tell Perl's parser that you really didn't mean a string there by adding the correct incantation:

#!/usr/local/bin/perl -wl
use strict;

use constant TESTVALUE => 1;

my %hash1 = (TESTVALUE()    => "OK");
my %hash2 = (123        => TESTVALUE);

my $hashValue;

print $hash1{1};
print $hash1{+TESTVALUE};
print $hash1{$hash2{123}};
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As you see, you do not have to use the comma instead of the double arrow - as long as Perl knows you mean the left side to be an expression, not a string. The same is true for hash lookup curlies. Note that prepending a plus sign won't work for the fat comma, though appending parens will work for the hash lookup. It's a bit confusing at first to remember which rules apply where, but you'll get used to them quick. In practice I find the specific rules and exceptions chosen to be the most convenient ones.

Makeshifts last the longest.