my $input = 'existing.txt';
my $output = 'new.txt';
open ( my $in, '<', $input ) or die "Cannot open file $input for reading: $!\n";
open ( my $out, '>', $output ) or die "Cannot open file $output for writing: $!\n";
print $out "<pre>\n";
while ( <$in> ) {
    print $out $_;
}
print $out "\n</pre>\n";
close $out or warn "Error writing $output: $!\n";
close $in;