Labelling groups (with (?pattern) regex) showed me what was happening. It is all clear now.

Many thanks to everyone, specially BillKSmith!

I was having a trouble when using $1, $2... because I didn't know they exact behaviour.

The problem I had was with the parenthesis inside $ip4 regex, causing $2 to have an ip address,
$3 had the match inside the parenthesis in $ip4, and the prefix I expected put into $4:

# Example of my debug output:

line = '*> 177.101.16.0/21  200.19.74.230   0    200'
matches: $1:'*> ' $2:'177.101.16.0' $3: '16.' $4:'/21'
[download]

Now I understand how $1, $2, $3... are set.
Thank you all, Monks! =D