I gone through the script but not able to understand

This code reads the input file and splits the lines into variables using unpack. It prints out the variables using pack

# loop over all the records in the input file
    while ( $record = <INPUT_FILE> ) {
        chomp($record);
        $input_record_count++;
       
        for (substr($record, 0, 1) ) {
            if    (/H/)  { &H_header($record); }     
            elsif (/C/)  { &C_claim($record); }     
            elsif (/P/)  { &P_provider($record); }     
            elsif (/D/)  { &D_detail($record); }         
            elsif (/M/)  { &M_message($record); }             
        }
        
    }# end-while
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The code is commented like this

    
sub H_header {  
    # unpack the record into local fields     
    # Write the EOB record
}

sub D_detail {    
    # parse the input record into different individual fields
    # write the detail service record
}
    
sub C_write_claims {
    foreach (@claim_recs) {    
        # parse the input record into different fields    
        # write the claim record
    }
}
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What are the changes you want to make ?

poj