open FH, "<infile.txt" or die $!;
open OUT, ">outfile.txt";
while (<FH>)
          {
          if (/(\S+):(\S+).*\n/)
               {
               $var1 = "$1";
               $var2 = "$2";
               print OUT "$var1_$var2\n"
               }
        	elsif (/(.*)\n/)
        		{
        		print OUT "$1\n";
               }
            }
     close FH;