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#!/usr/bin/perl -w
use strict;
...
    );
  exit;
}

The arguments to C<traverse_sequences> are the two sequences to
traverse, and a callback which specifies the callback functions, like
...
Callbacks are invoked with at least the indices of the two arrows as
their arguments.  They are not expected to return any values.  If a
callback is omitted from the table, it is not called.

sub diff
{
...
    &$match();
    return wantarray ? @$retval : $retval;
}