These two snippets run from the command line show the difference between just declaring a scalar and initialising it to an empty string. Running under use warnings, the first snippet generates the message, the second doesn't as the string has been initialised.

$ perl -e '
> use strict;
> use warnings;
> my $string;
> print qq{>$string<\n};'
Use of uninitialized value in concatenation (.) or string at -e line 5
+.
><
$ perl -e '
> use strict;
> use warnings;
> my $string = q{};
> print qq{>$string<\n};'
><
$
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I hope this illustrates the difference.

Cheers,

JohnGG