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copied!<p>The problem is Perl can't make some optimization which is common in many compiled programming languages. As <a href="https://stackoverflow.com/users/589924/ikegami">ikegami</a> pointed out, each while cycle you copy data out from hash and you also do many unnecessary hash look-ups. There is some benchmark code you can play around.</p> <pre><code>#!/usr/bin/env perl use 5.10.0; use strict; use warnings; use Benchmark qw(:hireswallclock :all); use Clone qw(clone); my $data = { map +( $_ => { students => { map +( $_ => undef ), 103991 .. 104010 } } ), 1 .. 70000 }; my $college_data_hr; sub sum_time { my $t = shift; $t = timesum( $t, $_ ) for @_; return $t; } sub my_cmp_these { my %bench = @_; my %times; for ( 1 .. 10 ) { push @{ $times{$_} }, do { $college_data_hr = clone($data); timeit( 1, $bench{$_} ); } for keys %bench; } $_ = sum_time(@$_) for values %times; cmpthese( \%times ); } my_cmp_these( orig_while => sub { while ( my ( $college_code, $college_info_hr ) = each( %{$college_data_hr} ) ) { while ( my ( $student_num, $student_info_hr ) = each( %{ $college_info_hr->{'students'} } ) ) { if ( $student_num < 104000 ) { ## Delete the info of students before 2004. delete $college_info_hr->{'students'}{$student_num}; } } } }, new_while => sub { while ( my ( undef, $college_info_hr ) = each( %{$college_data_hr} ) ) { my $s = $college_info_hr->{'students'}; while ( my ( $student_num, undef ) = each(%$s) ) { if ( $student_num < 104000 ) { ## Delete the info of students before 2004. delete $s->{$student_num}; } } } }, orig_foreach => sub { foreach my $college_code ( keys %$college_data_hr ) { foreach my $student_num ( keys %{ $college_data_hr->{$college_code}{'students'} } ) { if ( $student_num < 104000 ) { ## Delete the info of students before 2004. delete $college_data_hr->{$college_code}{'students'} {$student_num}; } } } }, new_foreach => sub { foreach my $college_info ( values %$college_data_hr ) { my $students = $college_info->{'students'}; delete @$students{ grep $_ < 104000, keys %$students }; } }, ikegami_foreach => sub { for my $college_code ( keys %$college_data_hr ) { my $students = $college_data_hr->{$college_code}{students}; delete @$students{ grep $_ < 104000, keys %$students }; } }, ); </code></pre> <p>Results on mine notebook:</p> <pre><code> s/iter orig_while new_while orig_foreach ikegami_foreach new_foreach orig_while 1.56 -- -25% -31% -35% -40% new_while 1.17 33% -- -8% -14% -21% orig_foreach 1.08 44% 8% -- -6% -14% ikegami_foreach 1.01 54% 16% 7% -- -8% new_foreach 0.927 68% 26% 16% 9% -- </code></pre> <p>Result for <code>248,000</code></p> <pre><code> s/iter orig_while new_while orig_foreach ikegami_foreach new_foreach orig_while 6.19 -- -27% -30% -33% -38% new_while 4.54 36% -- -5% -8% -16% orig_foreach 4.31 44% 5% -- -4% -11% ikegami_foreach 4.16 49% 9% 4% -- -8% new_foreach 3.83 62% 19% 13% 9% -- </code></pre>

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