abaugher@bannor> cat 989705.pl 
#!/usr/bin/env perl
use Modern::Perl;
use Benchmark qw(:all);
use Regexp::Assemble;

my %h = map { $_ => uc } ( 'aa' .. 'zz' );
my $s = `cat bigfile`; # 8MB file

say "Testing with @{[-s 'bigfile']} byte file and @{[ scalar keys %h ]} patterns";

cmpthese( 10, {
        'forloop' => \&forloop,
        'pipes'   => \&pipes,
        'regexpa' => \&regexpa,
});

sub forloop {
        $s =~ s/$_/$h{$_}/g for keys %h;
}

sub pipes {
        my $p = join '|', keys %h;
        $s =~ s/($p)/$h{$1}/g;
}

sub regexpa {
        my $p = Regexp::Assemble->new->add(keys %h)->re;
        $s =~ s/($p)/$h{$1}/g;
}
abaugher@bannor> perl 989705.pl                               
Testing with 8560854 byte file and 676 patterns
              Rate forloop regexpa   pipes
forloop 9.75e-02/s      --    -96%    -99%
regexpa     2.40/s   2364%      --    -74%
pipes       9.08/s   9213%    278%      --