I will attempt to clarify my previous answer, given questions sent to me offline.

The question is: Why perl generates 'Use of uninitialized value in scalar assignment' only for the second call of foo and not for the both calls? What's the difference?

The first call specifies a key. The value is undefined, but the key is not.
The second one has an undefined key.

Hopefully this code will demonstrate the difference -- the only error generated is when the key is undefined. The value has nothing to do with the problem.

#!/usr/bin/perl

use strict;
use warnings;

my %bar = ();

$bar{not_existing} = 1;
$bar{"is_existing"} = 1;

my $bar1 = $bar{not_existing};
my $bar2 = $bar{"is_existing"};
my $bar3 = $bar{really_not_existing};

my $key4 = "value_not_existing";
my $bar4 = $bar{$key4};

my $key5;
my $bar5 = $bar{$key5};

exit;

__END__

C:\Steve\devcvs\mapgen>\steve\t\t2.pl
Use of uninitialized value $key5 in hash element at C:\steve\t\t2.pl l
+ine 19.
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