#!/usr/local/bin/perl -w

use strict;

# Read in file and build HoH
open INPUT, "<names.csv" or die "Blerch: $!\n";
my %existing_names;
foreach $line (<INPUT>) {
  my ($name,$surname)=split /,/, $line;
  $existing_names{$surname}{$name}++;
}  
close INPUT;
...
# Check and add new record
sub duplicate_record {
  
  my $name=shift;
  my $surname=shift;

  if (exists ($existing_records{$surname}{$name})) {
    do_complain_about_duplicates();
    return;
  }
 
  $existing_records{$surname}{$name} 
  
}