# before the loop
sub enquote {
  return { map { qq("$_") => undef } @_ };
}

my %attributes=(
  'fo:font-style' => enquote( qw(normal italic oblique backslant inherit ) ),
  'fo:fontvariant'=> enquote( qw(normal small-caps inherit ) ),
  'fo:fontweight' => enquote( qw(normal bold bolder lighter inherit 100 200 300 400 500 600 700 800 900) ),
  'fo:visibility' => enquote( qw(visible hidden collapse inherit) ),
);

# then, when you're inside the loop:
  for my $k (keys (%are)) {
    if ( exists( $attributes{$k} ) ) {
      if ( ! exists($attributes{$k}{$are{$k}}) ) {
        print FOUT $errorMessage;
      }
    }
  }