Example:

01 #!/usr/bin/perl
02
03 use strict;
04 use warnings;
05
06 my $testval;
07
08 my $newval = "$testval test";
09
10 exit;
11
12 __END__

C:\Steve\Dev\PerlMonks\P-2013-10-15@0712-Uninitalized>uninit.pl
Use of uninitialized value $testval in concatenation (.) or string at 
C:\Steve\Dev\PerlMonks\P-2013-10-15@0712\Uninitalized\uninit.pl line 8
.
[download]

Line 8 attempts to build a string using embedded translation of $testval which is, at that time, uninitialized.

So the error message should tell you where to look. All the usual caveats apply; depending on what you're doing, the actual error in Perl could be not exactly on the line where perl caught the error, but at least you have reduced it from a haystack to a handful of hay.

Good luck!