open my $infile, '<', @ARGV[0]
   or die ....

##</code><code>##

while ( my $line = <INFILE>) {
    chomp $line;
...

##</code><code>##

# 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 ...

##</code><code>##

given ( determine_type( $line ) ) {
   when { $PATIENT_ID } { process_patient( $line, \%data ); }
   when { $DOB }        { process_dob( $line, \%data ); }
...

##</code><code>##

# define the process_hash near the top
#
Readonly my %PROCESS => ( $PATIENT_ID => \&process_patient,
                          $DOB        => \&process_dob,
                          ...
                         );

# and use it in the loop
#
my $type = determine_type( $line );
$PROCESS{$type}->( $line, \%data );