Is it always zero at the beginning of the loop body, or on every other iteration?  In case of the latter, it could be because the conditions you're testing are such that $i gets decremented in the elsif branch every time it has become 1.  Heavily simplified, like this:

my $i=0;

foreach my $field (qw(foo bar baz quux)) {

    print "i=$i\n";
    
    if ($i == 0) {
        $i++;                 # increment
    }    
    elsif ($i == 1) {
     
        if (0) {
            if (0) {
                $i--;
            }    
        }
        else {
            $i--;             # decrement   
        }            
    }    
}

__END__
i=0
i=1
i=0
i=1
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