my @k = sort keys %the_hash;

my $pct = 10; # what percent to keep
#- WRONG -# for ( int(@k/$pct)-1..$#k ) { delete $the_hash{$k[$_]} }
for ( int( @k * ($pct/100).. ) )-1..$#k ) {
   delete $the_hash{$k[$_]} 
}
# Now %the_hash only contains the top 10%