It skips the warning only if the uninitialized value is at the beginning of the string, and only in Perl 5.6.1.

Using Perl 5.8, it does warn you all right.

 1 #!/usr/bin/perl -w
 2 use strict;
 3 
 4 print "Perl $] \n";
 5 my %hash = ( a => undef, b => 'foo', c => undef );
 6 
 7 print "$_ => $hash{$_}\n" for keys %hash;
 8 print "$hash{$_} <= $_\n" for keys %hash;
 9
10 my $var = undef;
11 print "$var <= at the beginning\n";
12 print "in the middle =>  $var <= \n";
__END__

Perl 5.006001
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 7.
a =>
b => foo
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 7.
c =>
 <= a
foo <= b
 <= c
 <= at the beginning
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 12.
in the middle =>   <=

Perl 5.008
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 7.
c =>
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 7.
a =>
b => foo
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 8.
 <= c
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 8.
 <= a
foo <= b
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 11.
 <= at the beginning
Use of uninitialized value in concatenation (.) or string at hash_prob
+.pl line 12.
in the middle =>   <=
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