$ cat foo.pl
use strict;
use warnings;

my ($unused, $i, $j);
$i = $j; # uninitialized
__END__
$ perlcritic -3 foo.pl
..
"$unused" is declared but not used. at line 4, column 1.  Unused variables clutter code and make it harder to read.  (Severity: 3)
..

##</code><code>##

$ perl -wle 'my $i; print $i'
Use of uninitialized value in print at -e line 1.
$ perl -wle 'my $i; $i += $i'
Use of uninitialized value in addition (+) at -e line 1.
$ perl -wle 'my $i; if ($i>0) {}'
Use of uninitialized value in numeric gt (>) at -e line 1.