split //, $_
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$explicit[0] => { $explicit[1] => 1 };
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I don't believe this would work. Firstly,

Doesn't matter, poulate @explicit however you like

Secondly, Will ensure the store(i.e the key) will only have the last product read from the input.

Is this important?

This is how you're supposed to use map, to create lists -- if you're not doing that stick with foreach (or to save memory stick with while)

If all you want is to treat it like foreach, you can use it in void context, but its not recommended, it clouds the purpose, and on older perls its not efficient

#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd pp /;

open my($fh),'<', \<<'__FUDGE__' or die;
STORE     PRODUCT_ID
====================
Sydney    12
Sydney    14
Canberra  12
Canberra  18
__FUDGE__

my %hash;
$hash{(split)[0]}{(split)[1]} = 1 while <$fh>;
seek $fh, 0, 0;

dd\%hash;
undef %hash;

map {
    my @explicit  = split ' ', $_;
    2 == @explicit ## ====
        and $hash{ $explicit[0] }{ $explicit[1] }++;
    ();
} <$fh>;

dd\%hash;
__END__
$ perl fudge
Use of uninitialized value in hash element at fudge line 16, <$fh> lin
+e 6.
{
  "====================" => { "" => 1 },
  "Canberra" => { 12 => 1, 18 => 1 },
  "STORE" => { PRODUCT_ID => 1 },
  "Sydney" => { 12 => 1, 14 => 1 },
}
{
  Canberra => { 12 => 1, 18 => 1 },
  STORE    => { PRODUCT_ID => 1 },
  Sydney   => { 12 => 1, 14 => 1 },
}
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Is this important?

Very important!