my $inp = "abc.xml";
my $out = "new_abc.xml"; # or maybe some more meaningful name...

open( INP, $inp ) or die "$inp: $!" );
open( OUT, $out ) or die "$out: $!" );

my @array = <INP>;  # read all input lines into @array

# ... $xml_add has to come from somewhere ...

$array[$#array] = $xml_add; # replace last line

print OUT join "", @array;