I would declare it only once, like this:

#!/usr/bin/env -S perl -n
use strict;
use warnings;

# USAGE:  perl np.pl <np.dat
our $total;

BEGIN{
    $total = 0;
}

$total += $_;

END{
    print $total
}
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This seems less verbose while retaining the clarity but it is all subjective, of course.