Sorry to mislead you.

I just changed the SYBASE variable to something, in that case 'as'.
So the error message shows that the original setting of the variable is used by DBD::Sybase and not the one in the BEGIN block.

I tried both in my BEGIN block, 'c:\w03g\sybase\r120500' as well as '/w03g/sybase/r120500'.

Both led to the same error.
The system('echo %SYBASE%'); shows that the variable is set correctly in the BEGIN block, DBD::Sybase just ignores this setting and uses the value set in the shell.