#!/usr/bin/perl -w
use strict;
use warnings;
my $a = 'a';
my $b;
my $c = 'c';
print "variablen:"
.$a
.$b
.$c
;
my %hash = (
        a => 'a',
        c => 'c',
);
print "variablen:"
.$hash{a}
.$hash{b}
.$hash{c}
;

Use of uninitialized value $b in concatenation (.) or string at test.pl line 7.
Use of uninitialized value in concatenation (.) or string at test.pl line 16.
variablen:acvariablen:ac