18:14 >perl -wE "my $s = qq[\n\n\n]; my $t = $s =~ s{$}{}gmr; say $s eq $t;"
1

18:14 >

##</code><code>##

#! perl
use strict;
use warnings;

my $s = "a\n\n\nb";
my $t = $s =~ s{^\s*?$}{}gmr;

printf "%s\n", $s eq $t ? 'success' : 'fail';
print  ">$s<\n";
print  "[$t]\n";

##</code><code>##

18:29 >perl 902_SoPW.pl
fail
>a


b<
[a

b]

18:29 >

##</code><code>##

...
Guessed: match at offset 0
   2 <a%n> <%n%nb>           |  1:MBOL(2)
   2 <a%n> <%n%nb>           |  2:MINMOD(3)
   2 <a%n> <%n%nb>           |  3:STAR(5)
   2 <a%n> <%n%nb>           |  5:  MEOL(6)
   2 <a%n> <%n%nb>           |  6:  END(0)
Match possible, but length=0 is smaller than requested=1, failing!
                                  POSIXD[\s] can match 1 times out of 1...
   3 <a%n%n> <%nb>           |  5:  MEOL(6)
   3 <a%n%n> <%nb>           |  6:  END(0)
Match successful!
...