It actually gets weirder than that -- the . (concatenation) operator has higher precedence than a hash's auto-stringification. This confused a somewhat baffling bug for a few hours recently... 
use Data::Dumper;

$x{a}     = "bare a";
$x{"a.b"} = "quoted a.b";
$x{a.b}   = "unquoted a.b";
print Dumper(\%x);'

__DATA__
$VAR1 = {
          'ab' => 'unquoted a.b',
          'a.b' => 'quoted a.b',
          'a' => 'bare a'
        };

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In reply to Re^3: Numeric as hash key returns undef... by Anonymous Monk
in thread Numeric as hash key returns undef... by journey

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