If anyone can think of a better title, /msg me and I'll change it.

Probably another of my "can't see the wood for the trees" questions, but why does this work:

P:\test>perl -wl
my $b .= 'fred' . 'bill';
print $b;;
^Z
fredbill
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And this:

P:\test>perl -wl
my $a = 'a' x 10;
my $b;
$b .= substr( $a, $_, 10-$_ ) . '-' x $_ for 0 .. 9;
print $b;
^Z
aaaaaaaaaaaaaaaaaaa-aaaaaaaa--aaaaaaa---aaaaaa----aaaaa-----aaaa------
+aaa-------aa--------a---------
[download]

But not this?

P:\test>perl -wl
my $a = 'a' x 10;
my $b .= substr( $a, $_, 10-$_ ) . '-' x $_ for 0 .. 9;
print $b;
^Z
Use of uninitialized value in print at - line 3.
[download]