If there were repeated, contiguous newlines, that would only avoid replacing the final newline.

c:\@Work\Perl\monks>perl -wMstrict -le
"my $line = qq{HeHi\n\n\nHo};
 print qq{[$line]};
 ;;
 $line =~ s/Hi\s*(?=\n)//;
 print qq{[$line]};
"
[HeHi


Ho]
[He
Ho]
[download]