I am not quite sure about the requirements. If all that this needed is to put blanks for cols 1 and 2 when they are the same as the previous line, then..perhaps..???

#!/usr/bin/perl
use strict;
use warnings;

my $prev_line = <DATA>;
$prev_line =~ tr/:/ /;  # BTW, tr/// is really fast
print $prev_line;

while (my $this_line = <DATA>)
{
   $this_line =~ tr/:/ /;
   my ($prev_cols ,$prev_file_name) = $prev_line =~ /^\s*(.*?)(\S+)\s*
+$/;
   my ($this_cols, $this_file_name) = $this_line =~ /^\s*(.*?)(\S+)\s*
+$/;
  
   if ($prev_cols eq $this_cols)
   {
       $this_cols =~ s/\S/ /g;
   }
   print $this_cols, $this_file_name,"\n";
   $prev_line = $this_line;
}

=Prints
ULTRIX                      CW18.72.0.3          IP-HOST1.log
DEC_DECSTATION              CW180.72.0.3         IP_HOST_AL.log
DEC_DECSTATION_ADDR         CW180.72.0.3         IP_HOST_al2.log
FOR_VISITORS23_HOST         HL617.253.1101.2     IP_HOST_hostinfo.log
FOR_VISITORS24_HOST         HL617.253.1101.2     IP_HOST_hostinfo2.log
FOR_VISITORS25_HOST         HL617.253.1101.2     IP_HOST_webform3.log
                                                 IP_HOST_webform4.log
FOR_VISITORS25_HOST         HL617.253.1201.2     IP_HOST_webform4.log
=cut


__DATA__
ULTRIX              :       CW18.72.0.3          IP-HOST1.log
DEC_DECSTATION      :       CW180.72.0.3         IP_HOST_AL.log
DEC_DECSTATION_ADDR :       CW180.72.0.3         IP_HOST_al2.log
FOR_VISITORS23_HOST :       HL617.253.1101.2     IP_HOST_hostinfo.log
FOR_VISITORS24_HOST :       HL617.253.1101.2     IP_HOST_hostinfo2.log
FOR_VISITORS25_HOST :       HL617.253.1101.2     IP_HOST_webform3.log
FOR_VISITORS25_HOST :       HL617.253.1101.2     IP_HOST_webform4.log
FOR_VISITORS25_HOST :       HL617.253.1201.2     IP_HOST_webform4.log
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