From Programming Perl, pg. 108 under 'Comma Operator' (emphasis mine):

The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. It also forces any identifier to its immediate left to be interpreted as a string.

TESTVALUE to the left of '=>' is being treated as a string literal. You need to replace the '=>' with a comma.

Also, a bareword inside braces is considered a string literal unless you tell perl to evaluate it by prepending a '+'. So $hashValue = %hash1->{TESTVALUE}; should be $hashValue = %hash1->{+TESTVALUE};

Instead of writing %hash1->{1}, you could write $hash1{1}.

#!/usr/local/bin/perl -w

use strict;

use constant TESTVALUE => 1;

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

my $hashValue;

# Try 1
$hashValue = $hash1{1};
# Try 2
$hashValue = $hash1{+TESTVALUE};
# Try 3
$hashValue = $hash1{$hash2{123}};
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Update:Fixed %hash2->{123} in last test.

---
print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';
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