use strict;
use warnings;


my @new_lines = some_function; # something that retrieve new lines

my $file_path = 'file_path.ext';

open my $orig_file, '<', $file_path or die "unable to open '$file_path' for read!";
my @orig_lines = <$orig_file>;

close $orig_file;

#reopen it wiping the content
open my $new_file, '>', $file_path or die "unable to open '$file_path' for write!";

#print first new get lines and then old ones
print $new_file $_ for @new_lines,@orig_lines;