use strict;
use warnings;

my %foo;
my %bar;
$bar{'one'} = 1;
my ($x,$y);
$x = "$bar{'one'} $foo{'one'}";
$x = "$y $foo{'one'}";
$x = "xxx $foo{'one'}";
__END__
Use of uninitialized value in concatenation (.) or string at /tmp/p line 10.
Use of uninitialized value $y in concatenation (.) or string at /tmp/p line 11.
Use of uninitialized value in concatenation (.) or string at /tmp/p line 11.
Use of uninitialized value $foo{"one"} in concatenation (.) or string at /tmp/p line 12.