It's my understanding that back references (\1) are a bit slow. What follows are solutions that don't use back references. You'll have to benchmark them to see if they're faster. 

#my @chars = grep !$seen{$_}++, $text =~ /./g;
my @chars = 'a'..'z';

my ($re) = map qr/$_/,
           join '|',
           map "(?<=${_}{3})$_+",
           map quotemeta,
           @chars;

$text =~ s/$re//g;

[download]
#my @chars = grep !$seen{$_}++, $text =~ /./g;
my @chars = 'a'..'z';

my ($re) = map qr/$_/,
           join '|',
           map "${_}{4,}",
           map quotemeta,
           @chars;

$text =~ s/($re)/substr($1,0,3)/eg;

[download]
#my @chars = grep !$seen{$_}++, $text =~ /./g;
my @chars = 'a'..'z';

my ($re) = map qr/$_/,
           join '|',
           map "${_}{4,}",
           map quotemeta,
           @chars;

$text =~ s/$re/substr($text,$-[0],3)/eg;

[download]

Update: Fixed bug identified in reply.

In reply to Re: More efficient way to truncate long strings of the same character by ikegami
in thread More efficient way to truncate long strings of the same character by kurtis

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