grep in scalar context returns the number of matches. To get the first match, you can use the following:

my ($i1) = grep { $array[$_] == 1 } 0..$#array;
my ($i2) = grep { $array[$_] == 2 } 0..$#array;
my ($i3) = grep { $array[$_] == 3 } 0..$#array;
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Might be worth creating a lookup table.

my %lookup = map { $array[$_] => $_ }  0..$#array;

my $i1 = $lookup{1};
my $i2 = $lookup{2};
my $i3 = $lookup{3};
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