Here's a working example. It does return the id either of the new record or the existing record. Unfortunately it also appears to change the auto-increment value with every update (even when no new records are inserted) so id's may not be continuous.

From the mysql docs :  In general, you should try to avoid using an ON DUPLICATE KEY UPDATE clause on tables with multiple unique indexes.

#!\C:\Strawberry\perl\bin\perl
use strict;
use warnings;
use DBI;

# CREATE TABLE test2 (
#  id int(11) NOT NULL AUTO_INCREMENT,
#  f1 varchar(45) DEFAULT NULL,
#  PRIMARY KEY (id),
#  UNIQUE KEY f1_UNIQUE (f1)
# ) ENGINE=InnoDB AUTO_INCREMENT=27 DEFAULT CHARSET=utf8;

my $table = 'test2';
my $col   = 'f1'; # unique key
my $entry = $ARGV[0] || 1;

#PERL DBI CONNECT
my $dbh = dbh();

my $sql = sprintf '
  INSERT INTO %s (%s) 
  VALUES (?) 
  ON DUPLICATE KEY 
  UPDATE id = LAST_INSERT_ID(id)',
  $dbh->quote_identifier($table),
  $dbh->quote_identifier($col) ;
  
my $sth = $dbh->prepare($sql);
my $rv = $sth->execute($entry);
print $rv.' '.$dbh->last_insert_id(undef,undef,$table,$col);
    
sub dbh{
  my $database = "test"; my $user = "user"; my $pw = "password";
  my $dsn = "dbi:mysql:$database:localhost:3306"; 
  my $dbh = DBI->connect($dsn, $user, $pw, 
     { RaiseError=>1, AutoCommit=>1 } );
  return $dbh;
}
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The last_insert_id is only useful with auto-incrementing keys (and even then I find it rarely needed), but in any case auto-incrementing keys should never give a duplicate key error.