=> is sometimes called a "fat comma". A normal comma is , ... Try it.

use constant {
    NULL => 0,
    EINS => 1,
    ZWEI => 2,
};
my %english_translation = (
    NULL, "Zero",
    EINS, "One",
    ZWEI, "Two",
);
foreach ( keys %english_translation )
{
    print "$_ = $english_translation{$_}\n";
}
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From perlop:

The "=>" operator is a synonym for the comma, but forces any word (consisting entirely of word characters) to its left to be interpreted as a string (as of 5.001). This includes words that might otherwise be considered a constant or function call.

Constants are, in perl, implemented as subroutines which don't take an argument and return the constant's value. That means that

use constant {
    NULL => 0,
    EINS => 1,
    ZWEI => 2,
};
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has the same effect as saying

sub NULL { return 0; }
sub EINS { return 1; }
sub ZWEI { return 2; }
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After either method (use constant LIST or setting up subroutines) you can use the sub denominating bare-words in your program, which will be replaced with the associated values by calling their corresponding subroutine.

But! if you construct your hash as you did,

my %english_translation = (
    NULL    => "Zero",
    'EINS'  => "One",
    ZWEI()  => 'Two',
);
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only the key ZWEI will be resolved as a call of a function, because, as per the above snippt from perlfunc, the "=>" (i.e. "fat comma") operator forces any word (consisting entirely of word characters) to its left to be interpreted as a string, so saying EINS => 1 is exactly the same as saying 'EINS' => 1. The list operator "," (i.e. the normal comma) doesn't do that, so saying

my %english_translation = (
    NULL, "Zero",
    EINS, "One",
    ZWEI, 'Two',
);
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will call the functions associated with NULL, EINS and ZWEI and interpolate their results.

has the same effect as saying
sub NULL { return 0; }

or more accurate, with an empty prototype:
sub NULL() { return 0; }