... any regex ... either succeeds or it fails, so either all the match strings are defined, or none of them are.

A minor cavil: capture groups that are not required to match for an overall match will return undef in their respective $n capture variables and in list context.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $s = 'abc';
 ;;
 my @captures = $s =~ m{ (?: (a)(b)(c) | (w)(x)(y)) (z)? }xms;
 dd \@captures;
"
["a", "b", "c", undef, undef, undef, undef]
[download]

The  (?|pattern) construct available with Perl version 5.10 (see Extended Patterns in perlre) modifies this behavior in alternations, but you may still be left with undefs from other sources.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"use 5.010;
 ;;
 my $s = 'abc';
 ;;
 my @captures = $s =~ m{ (?| (a)(b)(c) | (w)(x)(y)) (z)? }xms;
 dd \@captures;
"
["a", "b", "c", undef]
[download]