I'm no expert in the specifics of the DBD::mysql build process, but this line in the make messages

dlltool --def mysql.def --output-exp dll.exp
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looks as if it's trying to create a dll.exp itself (from a mysql.def), which is then being referenced in the subsequent compile/link step.  In other words, I would not try to create a libmysql.a myself, but rather just rename the output of "reimp libmysql.lib" to mysql.def, and leave the rest to the build process as is...  Just an idea, though. Hope it helps.

(Be sure to also check whether the .def file does in fact contain the symbols that are being reported as "undefined"...)