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use List::Util qw( reduce );

...
   my ($ar) = @_;
   return reduce { $ar->[$b] > $ar->[$a] ? $b : $a } 0..$#$ar;
}

sub min_idx {
   my ($ar) = @_;
...
   for (1..$#$ar) { $idx = $_ if $arr->[$_] > $arr->[$idx]; }
   return $idx;
}

my $min_idx = min_index(\@a);