open FILE, "myfile.txt" or die "Couldn't open for read!";
# stick all lines of file into an array of strings
@lines = <FILE>;
close FILE;

open FILE, ">myfile.txt" or die "Couldn't open for write!";
foreach $line (@lines)
{
   # create a list of words in each line
   @words = split ' ', $line;
   
   # editing code goes here
   if( $words[1] eq "posp" )
   {
      $words[0] = "edited";
   }
   
   # editing is done, now write the line to the file
   print FILE $_." " foreach( @words );
   print FILE "\n";
}
close FILE;
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