c:\@Work\Perl\monks>perl -wMstrict -le
"for my $sr ('k\s', 'k\\s', 'k\\\\s', qr{ k \\ s }xms, qr{k\\s}xms) {
   my $s = 'remove back\slash';
   printf qq{'$s' %s -> }, ref $sr ? $sr : qq{'$sr'};
   $s =~ s{$sr}{xx}xmsg;
   print qq{'$s'};
   }
"
'remove back\slash' 'k\s' -> 'remove back\slash'
'remove back\slash' 'k\s' -> 'remove back\slash'
'remove back\slash' 'k\\s' -> 'remove bacxxlash'
'remove back\slash' (?^msx: k \\ s ) -> 'remove bacxxlash'
'remove back\slash' (?^msx:k\\s) -> 'remove bacxxlash'