Your script appears to be complaining about $ARGV[0] not having been initialized. The error you found doesn't really have anything to do with the concatenation operator itself. It's merely saying that one of the variables in the concatenation has yet to be initialized.

This happens when you run your script with the -w flag or "use warnings." It does this because you may have accidentally mispelled a variable name and rather than using the variable you expected, you're using an uninitialized variable which could cause wierd things to happen. Of course, "use strict" reduces that possibility as well (/me wondering what the implications of making this warning go away under use strict).

Since it's complaining about $ARGV[0] i have to wonder: did you supply a command line argument to your script?

All in all, it may be safer to use Getopt::Std to do your command line input.