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#!usr/bin/perl
use strict;
...
# write your new data on empty file
# close file
# done :D

An older style is to use a bareword as the filehandle, as

...
       or die "Can't open < input.txt: $!";

Then you can use FH as the filehandle, in close FH and <FH> and so on.
+ Note that it's a global variable, so this form is not recommended in
+ new code.

It is global to all the script you write so if anyone uses the same na
+me (IN or OUT in our example) those will clash with yours.

It is also harder to pass these variables to functions, than to do the
+ same with regular scalar variables.