Underscores in numbers are quite valid: see perldata: Scalar value constructors.

If you put a non-digit character in front of 512_x64, the problem goes away.

As a side issue relating to your code, there's no my in front of %h. If you add use 5.12.2; (or indeed any version >= 5.11), strictures are turned on automatically. They are not turned on simply by having that version running.

Here's my test:

#!perl

use 5.12.0;
use warnings;

my %h = ( 'a512_x64' => '24', 'world' => 'hello');

print $h{world}, "\n";   # line 5
print $h{a512_x64}, "\n"; # line 6
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which outputs:

C:\_\tmp>num_underscore_problem.pl
hello
24
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That's 5.12.0 running under Windows.