You're not showing us the code that contains the concatenation that produces the error.

First I disagreed, because it says "or string" in the warnings, but then I tested:

perl -wle 'my $a; $a =~ m/foo/'
Use of uninitialized value in pattern match (m//) at -e line 1.
[download]

So matching against an undefined value is a different warning, and you are right, the relevant code wasn't quoted in the original question, presumably.