One nice way to do this is to treat the hash as array, then take a slice from the array and reassign it. The program below demos this approach. 
#!/usr/bin/perl
use strict;
use warnings;

# Create a test hash.
my %test;
$test{$_}="$_-$_" for ('aa' .. 'zz');

# Grab N keys.
my $num_keys = 5;
%test = (%test)[0..$num_keys*2-1];

# Display the result
for (sort keys %test) {
  print "$_ = $test{$_}\n";
}

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In reply to Re: Delete all hash keys except for n of them by Molt
in thread Delete all hash keys except for n of them by Anonymous Monk

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