gistfile1.PL

#!/usr/bin/env perl

use strict;
use warnings;
use Benchmark qw/timethese cmpthese/;

my @lines;
my $word = 'a';
push @lines, $word x $_ for 1..10000;
@lines = (@lines, @lines);

cmpthese timethese 0, {
    one_liner => sub {
        return [ sort { $b->[1] <=> $a->[1] } map { [$_, length $_] } do { my %h; grep { $h{$_}++ == 1 } @lines } ]->[0][0];
    },
    fast => sub {
        my %h;
        my $maxstr = '';
        for (@lines) {
            $maxstr = $_ gt $maxstr ? $_ : $maxstr if $h{$_}++ == 1;
        }
        return $maxstr;
    },
    fastest => sub {
        my %h;
        my $maxstr = '';
        for (@lines) {
            $maxstr = length $_ > length $maxstr ? $_ : $maxstr if $h{$_}++ == 1;
        }
        return $maxstr;
    },
};

__END__
Benchmark: running fast, fastest, one_liner for at least 3 CPU seconds...
      fast:  4 wallclock secs ( 3.00 usr +  0.17 sys =  3.17 CPU) @  1.58/s (n=5)
   fastest:  3 wallclock secs ( 3.09 usr +  0.08 sys =  3.17 CPU) @  1.89/s (n=6)
 one_liner:  4 wallclock secs ( 3.11 usr +  0.36 sys =  3.47 CPU) @  1.44/s (n=5)
            Rate one_liner      fast   fastest
one_liner 1.44/s        --       -9%      -24%
fast      1.58/s        9%        --      -17%
fastest   1.89/s       31%       20%        --