G'day dd-b,

That's an indirect subroutine call. You can use this incantation to see how Perl parses your statement:

$ perl -MO=Deparse,-p -e 'my $map = match_maprow_by_hash $self( $first
+ );'
(my $map = $self->match_maprow_by_hash($first));
-e syntax OK
[download]

Compare that with these lines from perlref:

$objref = new Doggie( Tail => 'short', Ears => 'long' );

$objref = Doggie->new(Tail => 'short', Ears => 'long');
[download]

perlsub also has information about indirect calls.