#!usr/bin/perl
use strict;
use warnings;

my $infile = '/path/path/data.txt';

open( my $in, "<", $infile ) # use filehandles not bareword
    or die "Couldn't open $infile: $!";

chomp(my @lines = <$in>); # store data in an array in case you need them

close $in
    or warn "close failed: $!"; # close file

# empty file
# open file in write mode
# write your new data on empty file
# close file
# done :D