I'm not sure if it applies to everybody, but I had this problem because I use the MySQL packages provided by Fink, but had not upgraded Fink to Mac OS X 10.6.

As a result, all my Fink libraries, including mysqlclient.dylib are 32-bit only.

The /usr/bin/perl in Snow Leopard is 32-or-64 bit, but prefers 64 bit. By the time it's loaded, it's definitely 64 bit and all further (runtime) libraries linked in HAVE to be 64-bit.

My solution is twofold: firstly, upgrade Fink into 64-bit land by reinstalling from scratch, as per their instructions. Secondly, I've decided to use the official MySQL build rather than Fink's build.

That's the bottom-line, but here are some other useful tips:

• otool -L will tell you what other libraries a bundle depends on.
• file on an executable will tell you what architectures it's compiled for.

In this case:

$ otool -L blib/arch/auto/DBD/mysql/mysql.bundle
blib/arch/auto/DBD/mysql/mysql.bundle:
        /sw/lib/mysql/libmysqlclient.15.dylib (compatibility version 1
+6.0.0, current version 16.0.0)
        /usr/lib/libz.1.dylib (compatibility version 1.0.0, current ve
+rsion 1.2.3)
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, curre
+nt version 124.1.1)

$ file /sw/lib/mysql/libmysqlclient.15.dylib 
/sw/lib/mysql/libmysqlclient.15.dylib: Mach-O dynamically linked share
+d library i386

$ file blib/arch/auto/DBD/mysql/mysql.bundle 
blib/arch/auto/DBD/mysql/mysql.bundle: Mach-O universal binary with 3 
+architectures
blib/arch/auto/DBD/mysql/mysql.bundle (for architecture x86_64):      
+  Mach-O 64-bit bundle x86_64
blib/arch/auto/DBD/mysql/mysql.bundle (for architecture i386):  Mach-O
+ bundle i386
blib/arch/auto/DBD/mysql/mysql.bundle (for architecture ppc7400):     
+  Mach-O bundle ppc
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