If your really need the hash.. maybe this will work:

my @names_marks = ("cliff 76", "john 52", "keith 90", "rob 52");

my (%hash, @names, @marks);

foreach (@names_marks) {
  my ($name, $mark) = split / /, $_;
  $hash{$_} = [ucfirst($name), $mark];
}
## then sort by the $mark
foreach my $key (sort {$b->[1] <=> $a->[1]} keys %hash) {
  print "\$key: $key \$value: $hash{$key}->[0] $hash{$key}->[1]\n";
}
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but if you only want to sort, try the Efficient sorting using the Schwartzian Transform

my @names_marks = ("cliff 76", "john 52", "keith 90", "rob 52");

my @sorted_names = map { $_->[0] }
                   sort { $b->[1] <=> $a->[1] }
                   map { [$_, (split(/ /, $_))[1] ] }
                   @names_marks;
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update: fixed typo in code.

Update 2: fixed missing parens in second part of code.