or download this
( my $line = ( join "", <> ) ) =~ s/\n\s+//g;
print $line;
...
firstline54324563213612531436436463246324634secondlidfndsfdsfdsfdthird
+linedsfsdf
firstline54324563213612531436436463246324634secondlidfndsfdsfdsfdthird
+linedsfsdffourthlinedsfsdffifthlinedsfsdf