G'day Latnam,

Welcome to the monastery.

" If I enter a number that is not displayed I should see - Target is not found. So for instance the number 100 is not displayed. I see - The last occurrence of 100 is at index -1."

Your problem is with this condition:

if ($search) { ...
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Looking at the documentation for List::MoreUtils, under first_index, you'll see "Returns -1 if no such item could be found."; right beneath that, you'll see another function, last_index, which is probably what you want for "The last occurrence ...". [note corrected spelling]

There are two instances where $search might be FALSE: just a newline is entered at the prompt, chomp removes the newline, $search now equals ''; a zero and a newline are entered at the prompt, chomp removes the newline, $search now equals '0'. Given zero might be a perfectly valid number to search for, if you actually wanted to check that something (other than just a newline) was entered, length would be a better check (i.e. if (length $search) { ...).

Finally, writing List::MoreUtils::function_name is unnecessary: see how it's shown in the doco.

Putting all that together, here's all the elements I've discussed: you'll probably just want a subset ot these.

#!/usr/bin/env perl

use strict;
use warnings;

use List::MoreUtils qw{first_index last_index};

my @array_to_search = qw{ 0 2 4 2 0 };

print 'Search: ';
chomp( my $search = <STDIN> );

if (length $search) {
    my $index_1 = first_index { $_ eq $search } @array_to_search;

    if ($index_1 >= 0) {
        my $index_N = last_index { $_ eq $search } @array_to_search;

        print "First occurrence of '$search' at index: $index_1\n";
        print "Last occurrence of '$search' at index: $index_N\n";
    }
    else {
        print "'$search' not found.\n";
    }
}
else {
    print "Nothing to search for!\n";
}
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Some sample runs:

Search:  
Nothing to search for!
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Search: 100
'100' not found.
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Search: 0
First occurrence of '0' at index: 0
Last occurrence of '0' at index: 4
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Search: 2
First occurrence of '2' at index: 1
Last occurrence of '2' at index: 3
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Search: 4
First occurrence of '4' at index: 2
Last occurrence of '4' at index: 2
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@a = (12, 18, 56, 76, 99);
$in = <STDIN>;

for(0..$#a){
   die "found at $_\n" if $in == $a[$_];
}
print "not found\n";
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@a = (12, 18, 56, 76, 99);
$in = <STDIN>;
sub{die "found at $_\n" if $in == $a[$_]}->() for(0..$#a);
print "not found\n";
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@a = (12, 18, 56, 76, 99);
$in = <STDIN>;
for(0..$#a){
   $i = $_ if $in == $a[$_];
}
$i == 0 ? print "not found\n" : print "found at $i\n";
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this:

@a = (12, 18, 56, 76, 99);
$in = <STDIN>;
for(0..$#a){
   $i = $_ if $in == $a[$_];
}
$i == 0 ? print "not found\n" : print "found at $i\n";
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