use strict ; use warnings ;

my %h = ('this is a test' => 2,
         'is a test' => 2,
         'a test' => 1,
         'this is' => 1,
         'is a' => 1,
         'unique' => 1,
         'also Unique' => 3,
        ) ;

my @k = sort { length($b) <=> length($a) } keys(%h) ;

my $s = "\0". join("\0", @k)."\0" ;  # Assumption: "\0" doesn't appear in any key
study $s ;

foreach my $k (@k) {
  $s =~ m/(.)$k(.)/ ;
  if (($1 ne "\0") || ($2 ne "\0")) { delete $h{$k} ; } ;
} ;

print join(', ', map "'$_'", sort keys %h), "\n" ;