I take it you haven't got strictures (use warnings; use strict;) enabled.

Had you done so, you would probably have been able to tell that, in

my %cookie_hash = $IN->{cookies};
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, you are attempting to assign a hash ref. to a hash (perlref)... try either

my %cookie_hash = %{ $IN->{cookies} };
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or, the more usual,

my $cookie_hash = $IN->{cookies};
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...