# at the top
use Readonly;
Readonly my $PATIENT_ID => 0;
Readonly my $DOB        => 1;

# and in the main code ... 

my %data;
my $linetype = determine_type( $line );
if ( $linetype == $PATIENT_ID ) {
    process_patient( $line, \%data );
elsif ( $linetype == $DOB ) {
    process_dob( $line, \%data );
elsif ...