My guess would be that the Mac uses one byte to hold the ú, but RedHat is using two. My point being that your source file has the characters encoded differently, with RedHat using UTF8 and the Mac using some 1-byte codepage.

To test, create a plain text file on both systems that both contain the ú. Then do an 'od -x' on both of them and see how the character is encoded. My guess, again, is that it'll be one byte on the Mac and two bytes on the RedHat machine.

If that's the case, then your solution is to programmatically figure out what encoding scheme is being used "natively" on each system the perl script is being run on. I don't know how to do that, though.