Locate the key that matches the name Feynman.

my $surname = 'Feynman';
my @matching_ids =
    grep { $accounts{$_}{surname} eq $surname }
    keys(%accounts);
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Make sure @matching_ids has at least one matching record.

die("No records matching surname $surname\n") if !@matching_ids;
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Handle multiple matches as you see fit.

Get the transactions. They are located in

@{ $accounts{$id}{transactions} }
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