# always test for failure

open (FILEHANDLE, "+< $filename") || die "Can't open $filename in update mode: $!\n"; 

# read entire file into array of lines

@array = <FILEHANDLE>; 

# do stuff to the file here

# go back to beginning of file handle

seek(FILEHANDLE, 0, 0) or die ("Seek failed on $filename: $!\n"); 
print FILEHANDLE @array or die ("Print failed on $filename: $!\n"); 

# truncate the file so we don't have excess garbage at the end

truncate(FILEHANDLE, tell(FILEHANDLE)) or die ("Truncate failed on $filename: $!\n"); 
close (FILEHANDLE) or die ("Close failed on $filename: $!\n");