Personally, I'd use split for something like this...

my $count = 1;

while ( <DATA> ) {
    if ( m/^\^status/ ) {
        my @f = split;
        $f[17] = $count++;
        $_ = join( ' ', @f ) . "\n";
    }
    print;
}

__DATA__
^status f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 f15 f16 f17 123
+ 123
^status f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13 f14 f15 f16 f17 345
+ 123
[download]

But you could use a regexp...

s/\d+\s+(\d+)$/$count $1/;
[download]