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    my $N = shift;
    my $sqrt = sqrt( $N );
...
    return 1 if @factor == 1;
        # ... rest of the code
        return $f < $sqrt ? $f : undef;

    my $N = shift;
    my $sqrt = sqrt( $N );
...
    return 1 if @factor == 1;
        # ... rest of the code
        return $f < $sqrt ? $f : undef;

    my $N = shift;    
    my @factor = prime_factors( $N );
...
    return 1 if @factor == 1;
        # ... rest of the code
        return $f;