use List::Util 'max';
my $largest_val = max map { expensive_func($_) } @items;

##</code><code>##

my $largest_item = argmax { expensive_func($_) } @items;

##</code><code>##

my ($largest_item, $largest_val) = argmax { expensive_func($_) } @items;

##</code><code>##

sub argmax(&@) {
    my $index = undef;
    my $max   = undef;
    my $block = shift;
    for (@_) {
         my $val = $block->($_);
         if ( not defined $max or $val > $max ) {
             $max   = $val;
             $index = $_;
         }
    }
    return wantarray ? ($index, $max) : $index;
}

sub argmin(&@) {
    my $index = undef;
    my $min   = undef;
    my $block = shift;
    for (@_) {
         my $val = $block->($_);
         if ( not defined $min or $val < $min ) {
             $min   = $val;
             $index = $_;
         }
    }
    return wantarray ? ($index, $min) : $index;
}