use strict;
my $count = 1;

while (<>) {
    if ( m/^\^sip/ ) {
      s[                   ## No need to explicitly capture anything
           (?<=\s)         ## 2nd last num field cannot be 4 digits
           \d{1,3}         ## only this is replaced. 
           (?= \s+ \d+ $ ) ## Ensure there's one more before the EOS
      ][$count]x;
      $count++;
    } 
    print ;
}