It took me a second to understand that you are referring to the short-circuiting nature of //. If you needed the short-circuiting behaviour pre-5.10, here's one solution: 
sub first(&@) {
   my $cb = shift(@_);
   for (@_) {
      my $i = ref eq 'CODE' ? $_->() : $$_;
      my $rv;
      $rv = $cb->() for $i;
      return $i if $rv;
   }
   return;
}

my $theme = first { defined }
   \$global_config->{modes}{$current_mode}{theme},
   \$theme_config->{current_theme},
   \&ask_user,
   \'none';

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In reply to Re^3: Sequential defined-or operators by ikegami
in thread Sequential defined-or operators by naikonta

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