poir,
As others have pointed out, you probably need to do some type of "buffering." There is a slightly novel recipe (14.7) in the cookbook that outlines an alternative most askers of this question would probably like.

to paraphrase 14.7

#!/usr/bin/perl

use DB_File;

my $file = shift;
print "usage $0 filename" unless $file;

tie( @array, "DB_File", $file, O_RDWR, 0666, $DB_RECNO ) 
   || die "Cannot open file $file ($!)\n";

if( $array[1] =~ /word4/ ) {
    $array[0] = "this line now changed";
}

untie @array;
[download]

A good way to waste ... er learn some perl would be to benchmark this approach against one of the other buffering approaches.
-derby