You can speed this up considerably by transliterating everything you can and then only substituting characters that need it.

my $string = 'ÀÁÂÃÄÅàáâãäåÇçÈÉÊËèéêëÌÍÎÏìíîïÒÓÔÕÖØòóôõöøÑñÙÚÛÜùúûüÝÿýÆ
+æÞþÐðß';

print deaccent($string);

sub deaccent{
    my $phrase = shift;
    return $phrase unless ($phrase =~ m/[\xC0-\xFF]/);
    $phrase =~ tr/ÀÁÂÃÄÅàáâãäåÇçÈÉÊËèéêëÌÍÎÏìíîïÒÓÔÕÖØòóôõöøÑñÙÚÛÜùúûü
+Ýÿý/AAAAAAaaaaaaCcEEEEeeeeIIIIiiiiOOOOOOooooooNnUUUUuuuuYyy/;
    my %trans = (
        'Æ' => 'AE',
        'æ' => 'ae',
        'Þ' => 'TH',
        'þ' => 'th',
        'Ð' => 'TH',
        'ð' => 'th',
        'ß' => 'ss'
    );
    $phrase =~ s/([ÆæÞþÐðß])/$trans{$1}/g;
    return $phrase;
}

[download]

Benchmarking puts it at about 6 times the speed. Moving the hash assignment outside the sub speeds both up about the same amount, they stay about 6:1 ratio.

use Benchmark qw( cmpthese );

my $string = 'ÀÁÂÃÄÅàáâãäåÇçÈÉÊËèéêëÌÍÎÏìíîïÒÓÔÕÖØòóôõöøÑñÙÚÛÜùúûüÝÿýÆ
+æÞþÐðß';

cmpthese( -5, {
deaccent => sub {
    my $phrase = $string;
    return $phrase unless ($phrase =~ m/[\xC0-\xFF]/);
    $phrase =~ tr/ÀÁÂÃÄÅàáâãäåÇçÈÉÊËèéêëÌÍÎÏìíîïÒÓÔÕÖØòóôõöøÑñÙÚÛÜùúûü
+Ýÿý/AAAAAAaaaaaaCcEEEEeeeeIIIIiiiiOOOOOOooooooNnUUUUuuuuYyy/;
    my %trans = (
        'Æ' => 'AE',
        'æ' => 'ae',
        'Þ' => 'TH',
        'þ' => 'th',
        'Ð' => 'TH',
        'ð' => 'th',
        'ß' => 'ss'
    );
    $phrase =~ s/([ÆæÞþÐðß])/$trans{$1}/g;
    return $phrase;
},

deaccent2 => sub{

    my %acc = qw(
        À A Á A Â A Ã A Ä A Å A Æ AE 
        Ç C 
        È E É E Ê E Ë E 
        Ì I Í I Î I Ï I 
        Ð TH Ñ N 
        Ò O Ó O Ô O Õ O Ö O Ø O 
        Ù U Ú U Û U Ü U 
        Ý U Þ TH ß ss 
        à a á a â a ã a ä a å a æ ae 
        ç c 
        è e é e ê e ë e 
        ì i í i î i ï i 
        ð th ñ n 
        ò o ó o ô o õ o ö o ø o 
        ù u ú u û u ü u 
        ý y þ th ÿ y
    );
    my $text = $string;
    $text =~ s/(.)/$acc{$1}?$acc{$1}:$1/eg;
    return $text;
   },

});

[download]

Returns on my system:

             Rate deaccent2  deaccent
deaccent2  4316/s        --      -86%
deaccent  30859/s      615%        --

With data that has fewer accented characters, the disparity should grow much greater since it will short circuit if there are no characters to be transliterated.

In reply to Re^2: The Björk Situation by thundergnat
in thread The Björk Situation by SheridanCat

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