A slightly faster solution than using sort() or a for loop is to use List::Util::min:

my $href = { '6' => '154', '4' => '152', '1' => '80', '3' => '151', '8
+' => '150' }; 
use List::Util qw(min);
my $min = min keys %$href;
$href = {$min => $href->{$min}};
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