my $rec;

sysopen my $fh, "test.txt", O_RDWR | O_CREAT or die "$!";
sysread $fh, $rec, -s $fh;

my $offs = 0;
while( $offs < length $rec ) {
	my $next_eol_offs = index $rec, "\n", $offs;
	$next_eol_offs = length( $rec ) - 1 if $next_eol_offs == -1;

	my $str = substr $rec, $offs, $next_eol_offs - $offs + 1;

	# work on $str; note that it includes the newline

	substr( $rec, $offs, $next_eol_offs - $offs + 1 ) = $str;

	$offs += length $str;
}

seek $fh, 0, 0;
syswrite $fh, $rec;

close $fh or die "$!";