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

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

# at the top
use Readonly;
...
elsif ( $linetype == $DOB ) {
    process_dob( $line, \%data );
elsif ...

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

# define the process_hash near the top
#
...
#
my $type = determine_type( $line );
$PROCESS{$type}->( $line, \%data );