I would add that a quick check is to pop into the debugger. [our is used because the debugger has issues with my variables.]

c:\perl\perl>perl -de 0

Loading DB routines from perl5db.pl version 1.19
Editor support available.

Enter h or `h h' for help, or `perldoc perldebug' for more help.

main::(-e:1):   0
  DB<1> use strict

  DB<2> use warnings

  DB<3> our %hash = { 'key1' => 'val1', 'key2' => 'val2' };

  DB<4> x \%hash
0  HASH(0x1cbd91c)
   'HASH(0x1ca1f4c)' => undef
[download]

[Code entered this way in the debugger won't give the warning the others mentioned.]

Note here that %hash has a stringified hash as the key in it (and that key has no value). Since {...} returns a hashref here, and %hash = ... is expecting key/value pairs (or something else that looks like a hash), it grabs the hashref as the first key, stringifies it, and takes undef as the corresponding value.

Compare that to this:

  DB<5> %hash = ( 'key1' => 'val1', 'key2' => 'val2' );

  DB<6> x \%hash
0  HASH(0x1cbd91c)
   'key1' => 'val1'
   'key2' => 'val2'
  DB<7>
[download]

which is probably what you wanted.

For grins, try this:

print {"key"=>"value"}, "\n";
[download]

That's what a hashref looks like, stringified. Not terribly useful, but a recent Quiz of the Week might be an interesting detour.