Lexical $b is out of scope. see 

 
D:\temp>perl -MO=Deparse -e "my $b = 1 for 1 .. 10"
;
foreach $_ (1 .. 10) {
    my $b = 1;
}
-e syntax OK

D:\temp>

[download]

So, your code is the same as 

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

[download]

In reply to Re: Mysterious append failure. by ccn
in thread Mysterious append failure. by BrowserUk

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