Sweet!!! I just love this:

        my @c;
        for (my $i = 0; $i < length; $i++) {
            for (my $j = 2; $i + $j <= length; $j++) {
                push @c, substr $_, $i, $j;
            }
        }
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*Very* pretty - thank you so much! Exactly the kind of thing I was asking for.

Incidentally, Perl complains about your "abuse" of $_:

Attempt to free unreferenced scalar: SV 0x9730848, Perl interpreter: 0
+x970a008 at /tmp/perm2 line 17, <DATA> line 1.
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but that's easily fixed. Again, thank you - that's a really nifty way to build that permutation list!