#!/usr/bin/perl
use warnings;
use strict;
use Syntax::Construct qw( // );

use Benchmark qw{ cmpthese };
use List::Util qw{ first };
use Test::More;

sub f0 {
    my ($arg) = grep length, shift, 'default';
    $arg
}

sub f1 {
    my $arg = first { defined && length } shift, 'default';
    $arg
}

sub f2 {
    my $arg = defined $_[0] && length $_[0] ? shift : 'default';
    $arg
}

our @args = ( undef, '', 'fred', 1, 1.5 ) x 1000;

for my $arg (@args[ 0 .. 4 ]) {
    is f0($arg), f1($arg), "f0-f1-" . ($arg // 'undef');
    is f0($arg), f2($arg), "f0-f2-" . ($arg // 'undef');
}
done_testing();

cmpthese -1, {
    grep   => q[ f0( $_ ) for @args ],
    first  => q[ f1( $_ ) for @args ],
    simple => q[ f2( $_ ) for @args ],
};
[download]

ok 1 - f0-f1-undef
ok 2 - f0-f2-undef
ok 3 - f0-f1-
ok 4 - f0-f2-
ok 5 - f0-f1-fred
ok 6 - f0-f2-fred
ok 7 - f0-f1-1
ok 8 - f0-f2-1
ok 9 - f0-f1-1.5
ok 10 - f0-f2-1.5
1..10
         Rate  first   grep simple
first   570/s     --   -14%   -45%
grep    661/s    16%     --   -36%
simple 1036/s    82%    57%     --
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

Thanks choroba & soonix.

As choroba wrote, it is not needed. Analyzed from right to left, it constructs a two-element-list of the (possibly undefined) argument and the default value. Grep applies the length function and filters the list according to that. We take the first element of grep's result.

• length undef → undef (boolean false)
• length '' → 0 (boolean false)
• length 0 → 1 (boolean true)

(Dang it!) I meant to write: You are NOT doing enough work as you have missed the test for defined'ness.