Alternatively, you can refer to the values in the insert part with the values() function:

insert into  foo    (bar, baz) 
             values (?,   ?)
on duplicate key update
             bar = values(bar)
           , baz = values(baz)
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So your statement string would be:

my $stmt = 'INSERT INTO contact_form (' . join(',', keys %sql) . ') 
            VALUES (' . join(',', ('?') x keys %sql) . ') 
            ON DUPLICATE KEY UPDATE '. join(', ', map { "$_ = values($
+_)" } keys %sql);
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