Some small problems needs to be fixed. They're listed below, but I've updated the original as well (to do the impossible!).

1.

        @this = @last;
        @last = ();
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should be

        @last = @this;
        @this = ();
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2.

my @match = ( $$a1[$b1] );
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should be removed.

3.

@{$a1}[ ($b1-$longest+1), $b1 ]
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should be

@{$a1}[ ($b1-$longest+1) .. $b1 ]
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4. The warning can safely be silenced.