Sometimes, using our imense psychic powers, we can tell what the error message is without being told, but sometimes we get it wrong. You are best to just tell us. Copy and Paste is your friend. It is also good practise to include a sample script the reproduces the problem. Yours does not. I can't run your sample code and see the error. The following code does show warning and most likely it is the one that you see:

use strict;
use warnings;

my $input;
my $temp = 'more stuff';

print $input.$temp."\n";
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This shows the warning Use of uninitialized value in concatenation (.) or string.

None if this is pertinent to your question title however. What you are really asking is Is it better to interpolate variables into a string or concatenate them. There is no one answer and at the end of the day do what you can read best (Perl changes it all around internally anyway). However, you never need to do both so $input="$input"."$temp"; is bogus. That can better be written as $input = $input . $temp;, or even better as $input .= $temp;.