Here's one that does case-insensitive matching and preserves any capitalization.

It just needs slight modification to read and write files. :)

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1190261

use strict;
use warnings;

my $seq_txt_file = <<END;
  scooped up by
  social travesty
  without proper sanitation
END

my @seqs = sort { length $b <=> length $a }
  map s/^\s+|\s+$//gr,
  split /\n/, $seq_txt_file;
my $regex = do { local $" = '|'; qr/(@seqs)/i };

print s/$regex/ $1 =~ s!\h+!bbb!gr /ger while <DATA>;

__DATA__
Many of them are scooped up by chambermaids, thrown into bin bags and 
+sent off to landfill sites, which is a disaster for the environment a
+nd a social travesty given that many people around the world are goin
+g without proper sanitation.
Another test with capitals, a Social Travesty if I've ever seen one wi
+thout Proper Sanitation.
[download]