the code below is untested. mostly, i'm using it for illustration of the type of code you should be writing. use strict, warnings, and CGI. localize your typeglobs. test the return status of open and close. use split when it makes sense. take advantage of select to make printing easier.

#!/usr/bin/perl
use strict;      ## keeps you honest, makes you a better programmer
use warnings;    ## this too...
use diagnostics; ## for explanations of error an warning messages
require 5.006;


## use the html generation methods in the infamous CGI module
use CGI qw/:standard/;

my( $infile, $outfile ) = ( 'prevplanin.txt', 'prevplanout.htm' );
my ($varalias, $varsource, $vartarget);

## indent your code properly
{
  ## localize your typeglobs so you don't pollute the namespace.
  ## this is done by creating them with [local] inside a block
  local( *IN, *OUT );
  
  ## always test for failures on file open
  open( IN, "<", $infile ) or die "ERROR: can't open $infile: $!"
  open( OUT, ">", $outfile ) or die "ERROR: can't open $outfile: $!";

  ## select the filehandle, so you don't have to specify it when you p
+rint
  select OUT; 
  ## specify autoflush mode, so print output is line buffered
  $|++;
  
  # print your html header
  print start_html( 
    -title => 'PLAN VERIFICATION', 
    -bgcolor => "#FFFFFF", 
    -text => "#000000",
  );

  ## set your input record seperator. here i chose newline followed by
+ "alias "
  ## this way you can support multi-line input strings
  $/ = "\nalias ";
  
  ## for each record
  while( <IN> ) 
  {
    ## split the space-seperated fields of your record
    ## take a slice of the array [split] returns
    my( $alias, $source, $target ) = (split)[0,4,6];
    {
      ## use CGI's html generation to create your table
      print table(
        { -width="37%", -border="0", -cellspacing="1", -cellpadding =>
+ "0" },
        [
          Tr( 
            { -bgcolor => "#0099CC", }, 
            td( { -colspan => "2" }, 'alias retrieved' )
          ),
          Tr(
            td( 
              { -width => "11%", -bgcolor => "#CCCCCC" },
              b( 'alias' ),
            ),
            td(
              { -width => "89%", -bgcolor => "#FFCC99" },
              $alias,
            )
          ),
          ## repeat for $source and $target
          ## ...
          
          ## you don't need $i, you can use $. instead (current record
+ number)
        ]
      );

    }
    print OUT "</body></html>"
  }
  
  ## finish your page
  print end_html();

  ## remember to check return status of close, too
  close( IN ) or warn "Warning: can't close file: $!";
  close( OUT ) or warn "Warning: can't close file: $!";
  
  ## we're done with *OUT, select a valid filehandle
  select STDOUT;
}
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~Particle *accelerates*