# generally pointless, I would not use the mere existence of the key to mean anything; check it's value
if ( exists $hashtest{abc} and defined $hashtest{abc} ) {
  print qq{This is worthless IMO.\n};
}

##</code><code>##

if (defined $hashtest{abc}) {
  print qq{Better than 'exists'; check for the value being undef or not undef.\n};
}

##</code><code>##

if (defined $hashtest{abc} and $hashtest{abc}) {
  print qq{Also, better than 'exists' and when I want to make sure the value is not "falsey"\n};
}