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 => <=
