use strict;
use warnings;

<DATA>;
print ''.(chomp, s/\s*$/ /, $_).($_ = <DATA>, s/^\s*(?=\S)//, $_) while <DATA>;

__DATA__
Header line
First line part 1.
Part 2 of first line.
Second line parts one  
  and two.
Third line - second part empty.

Fourth
and last line