((this)?|(that)?|(third_thing)?)
...
... I don't even trust it. ... there's probably 4 capture groups created as a result.

Just as an aside, the  (?|(pat)|(te)|(rn)) "branch reset" pattern introduced with Perl version 5.10 will suppress the creation of a slew of captures in a case like this:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = 'apathetic';
 ;;
 my @captures = $s =~ m{ (pat) | (te) | (rn) }xms;
 dd \@captures;
 ;;
 @captures = $s =~ m{ (?| (pat) | (te) | (rn)) }xms;
 dd \@captures;
"
["pat", undef, undef]

["pat"]
[download]

See Extended Patterns in perlre.