If your program executes, that's a hint that this is just a warning, and not an error. Perl programs abort on errors (unless otherwise provided for).

Read the message - it is complaining about concatenation. More likely the error is in the print statement following. If $start_of_barrel_region is undefined, $s will be too, and so then $hash_plp_lbl_barrel_region_only{$s} will be undefined because the key is undefined. Once the loop body has executed once, the $s++ will make $s now be 1, and you won't see the warning again.

Use the debugger to inspect the values to make sure they are what you expect.