sub first (&@) {
  my $code = shift;

  foreach (@_) {
    return $_ if &{$code}();
  }

  undef; # <====== BUG?
}

##</code><code>##

D:\workspace> perl
D:\eclipse\workspace\BDC\lib> perl
use strict;
use warnings;
use Data::Dumper;
use List::Util;

my @list = 1..20;
my @first = List::Util::first { $_ == 21 } @list;
print Dumper( \@first );
if( @first ) {
  print "In scalar-context: true\n";
}
^D
$VAR1 = [
          undef
        ];
In scalar-context: true

##</code><code>##

D:\workspace>perl -MList::Util -le "print $List::Util::VERSION"
1.19