Please see my edited code above

Please have a look at How do I change/delete my post?, in particular, "It is uncool to update a node in a way that renders replies confusing or meaningless". But luckily I still had your original code sitting around:

sub get_max_index {
 my $i, $imax;
 for ($i=0; $i<@_; $i++)
 {
 $imax=$i if ($_[$i] > $_[$imax]);
 }
 return $imax;
}

my @arr = (1..10);
my $ans = get_max_index(@arr);
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Other than that, Paladin has already pointed out the issue with the second my declaration inside the loop in your updated code. I just wanted to add that in general, one should not combine my with an if statement modifier, as its behavior is undefined (see Statement Modifiers).