my %CALPHA = (

    'A' => [ 'wanker', 'tweet', 'bonk' ],

    'B' => [ 'ping', 'zilch' ],

 ...

);

my %DALPHA;

foreach my $k ( keys %CALPHA ) {

  foreach my $l ( @{$CALPHA{$k}} ) {

    $DALPHA{$l} = $k;

  }

}