Okay, good idea, graff. Here's what I did:

use strict;
use warnings;
use DBI;
use Try::Tiny;

$|=1;

my $redo;
my $dbh;
print 'mysql username: ';
chomp(my $username=<STDIN>);
print 'mysql password: ';
system('stty','-echo');chomp(my $pw=<STDIN>);system('stty','echo');
print $/;
sub reconn {
  $dbh = DBI->connect('dbi:mysql:scratch;mysql_enable_utf8=1',$usernam
+e,$pw,{mysql_auto_reconnect => 1}) or die $DBI::errstr;
}
reconn;
my $sth;

# MariaDB [scratch]> create table foo(bar int,baz int,primary key(bar)
+);
# Query OK, 0 rows affected (0.02 sec)
# 
# MariaDB [scratch]> insert foo(bar)value(1);
# Query OK, 1 row affected (0.01 sec)
# 
# MariaDB [scratch]> desc foo;
# +-------+---------+------+-----+---------+-------+
# | Field | Type    | Null | Key | Default | Extra |
# +-------+---------+------+-----+---------+-------+
# | bar   | int(11) | NO   | PRI | 0       |       |
# | baz   | int(11) | YES  |     | NULL    |       |
# +-------+---------+------+-----+---------+-------+
# 2 rows in set (0.00 sec)
# 
# # MariaDB [scratch]> select * from foo;
# +-----+------+
# | bar | baz  |
# +-----+------+
# |   1 | NULL |
# +-----+------+
# 1 row in set (0.00 sec)

$\=$/;

my $rows;
$dbh->ping or reconn;
$sth = $dbh->prepare("insert foo(bar)value(2);");
$redo = 1; while ($redo) {$redo=0; try {$rows=$sth->execute();} catch 
+{$redo = /deadlock found/i;}}
print "new: $rows";

undef $rows;
$dbh->ping or reconn;
$sth = $dbh->prepare("insert ignore foo(bar)value(1);");
$redo = 1; while ($redo) {$redo=0; try {$rows=$sth->execute();} catch 
+{$redo = /deadlock found/i;}}
print "old ignore: $rows";

undef $rows;
$dbh->ping or reconn;
$sth = $dbh->prepare("insert foo(bar)value(1) on duplicate key update 
+baz=1;");
$redo = 1; while ($redo) {$redo=0; try {$rows=$sth->execute();} catch 
+{$redo = /deadlock found/i;}}
print "old on duplicate key update: $rows";
[download]

And the results were:

new: 1
old ignore: 0E0
old on duplicate key update: 2
[download]

And row_count gives the same numbers (in these cases):

# MariaDB [scratch]> select * from foo;
# +-----+------+
# | bar | baz  |
# +-----+------+
# |   1 | NULL |
# +-----+------+
# 1 row in set (0.00 sec)
# 
# MariaDB [scratch]> insert foo(bar)value(2);
# Query OK, 1 row affected (0.00 sec)
# 
# MariaDB [scratch]> select row_count();
# +-------------+
# | row_count() |
# +-------------+
# |           1 |
# +-------------+
# 1 row in set (0.00 sec)
# 
# MariaDB [scratch]> insert ignore foo(bar)value(1);
# Query OK, 0 rows affected, 1 warning (0.00 sec)
# 
# MariaDB [scratch]> select row_count();
# +-------------+
# | row_count() |
# +-------------+
# |           0 |
# +-------------+
# 1 row in set (0.01 sec)
# 
# MariaDB [scratch]> insert foo(bar)value(1) on duplicate key update b
+az=1;
# Query OK, 2 rows affected (0.00 sec)
# 
# MariaDB [scratch]> select row_count();
# +-------------+
# | row_count() |
# +-------------+
# |           2 |
# +-------------+
# 1 row in set (0.00 sec)
[download]

Ah, but now I've tried an on duplicate key update where the update didn't actually change anything (the value to update to was there already), and select row_count() gave 0, but execute returned 1. So they're not always the same.

(The query was insert foo(bar)value(1)on duplicate key update baz=1; and the table already had 1 in each column.)

$_="msh210";$"=$\;@_=@{[split//,uc]}[2,0];$_="@_$\1";$\=$/;++$_[0]for$...1;print lc substr crypt($_,"@_"),1,6