my ($pos) = grep $input[$_] eq $val, 0 .. $#input;
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or create lookup table:

my ($first, $last) = ({}, {});
@$first{reverse @input} = reverse 0 .. $#input;
@$last{@input} = 0 .. $#input;

my $val = 'b';
print "Last index  : ", $last->{$val};
print "First index : ", $first->{$val};
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