$ perl -Mstrict -Mwarnings -MList::Util=min -E '
my @NI = ( 1, 1, 3, 4 );
my @NI_index = do {
   my $min = min @NI;
   grep { $NI[ $_ ] == $min } 0 .. $#NI;
   };

say qq{@NI_index};'
0 1
$