Why does the code below not work ?

For a number of reasons. But mainly because grep returns the value of $_ and because your hash assignment is completely wrong?

Why not try turning on use strict and use warnings and see what it says.

You probably want something like this (untested):

my %a = %$a;
my @keys = grep { ! /le/ } keys %$a;
my %c;
@c{@keys} = @a{@keys};
my $c = \%c;
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