if (/(\S+):(\S+).*\n/)
     {
     $var1 = "$1";
     $var2 = "$2";
     print OUT "$var1_$var2\n"
     }
  elsif (/(.*)\n/)
      {
      print OUT "$1\n";
      }
     }
[download]

Ok, once you get that working (i.e., fix  $var1_ problem), try reducing the big if-statement to a single  s/// substitution (untested):
    s{ (\S) : (\S) }{${1}_$2}xmsg;
(which just replaces every : with an _), followed by a print of the (possibly altered) line (held in  $_ in this code) to the output filehandle.

Update: This reply may have been superseded by your intervening post, but what the heck...