One would think so, eh? But I actually tested that before I posted:

#!/usr/bin/perl -w
use strict;
my %h;
$h{okay} .= "Hello";  # No warning (for me, at least)
warn "----\n";
$h{warn}= $h{warn} . "Hello";  # Gives a warning
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produces

----
Use of uninitialized value in concatenation (.) at line 6.
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But perhaps this isn't true in some older versions of Perl (I only tested v5.6.0).

        - tye (but my friends call me "Tye")