A more succinct (but semantically equivalent) version of:

foreach (@myarray) {
      $myhash{$_} = 1;
}
[download]

is

$myhash{$_}++ for @myarray;
[download]

I once heard (somewhere) that doing

if ($myhash{'blah'}) {
    do_something();
}
[download]

(i.e. refering to a key in a hash) causes that key to come into existence, so the above expression may always evaluate as true. I haven't been able to prove this, but I think what you want is the exists function:

if (exists($myhash{'blah'})) { 
  # Do something here
}
[download]

I quite like using 'exists', it makes code far more readable.