Unless there's a really compelling reason to do this in a single regex I wouldn't.

my %results;
for my $tok ( split /\s+/, $input ) {
  $results{ $1 } = $2 if $tok =~ /^ (.) = (\d+) $/x;
}

for my $k ( qw( a b c ) ) {
  print "$k is ", exists $results{ $k } ? $results{ $k } : 'missing', 
+"\n";
}
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Update: Just to clarify, eschewing explicit iteration to get away with just a single regex doesn't really gain you anything and (arguably) makes your code harder to read/maintain/understand ("OK, it's a for loop" vs "ZOMGWTFBBQ is this ?<a . . . grah, let me open perlre and try and remember")