You could join the array into a scalar with an unusual character as a separator, then use a match.

$foobar='D';
$sep = chr(1);

@array = ('A','B','C','D');
if (($sep.join($sep,@array).$sep) =~ m/$sep$foobar$sep/) {
   print "Found a $foobar in \@array!";
}
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The $sep on either side of the join() ensures that any potential match will begin and end with chr(1). You can pick other separator characters, as long as you can be sure they will not exist in your array (chr(1) is a pretty unlikely thing to find).

Of course, it would be better to use hash-keys if you can...

$foobar='D';
%hash = ( 'A' => undef, 'B' => undef, 'C' => undef, 'D' => undef );
print "Found a $foobar in \%hash" if exists $hash{$foobar};
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If you're avoiding multiple loop passes for performance, then you could use a one-time loop to convert the array to a hash:

@array = ('A','B','C','D');
for (@array) {
  $hash{$_} = undef;
}

#now we have the same hash as in the previous example.
#so, you can do this repeatedly with out a hit:

$foo = 'D';
if (exists $hash{$foo}) {
   print "There was a $foo in the array\n";
}
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