my %testar = (
  'reg1' => { 
    'exp' => qr~\Q/var/logs/logger/\E(\d*)/(\d*)~, 
    'desc' => 2,
  },
  'req2' => {
    'exp' => qr~...~,
    'desc' => 1,
  },
);

# entweder ueber ein explizites array:
my @fields = $string =~ $testar{req1}->{exp};
my $wantedField = @fields[ $testar{req1}->{desc} - 1 ];

# oder kuerzer, ohne das explizite array
my $wantedField = ( $string =~ $testar{req1}->{exp} )[ $testar{req1}->{desc} -1 ];