...but note that exists does not test what you asked for in the subject line, i.e. if a subhash exists — it just tests whether the respective hash key exists, or more precisely, whether the respective hash element has ever been initialized (with whatever value, including undef).

$data->{foo} = "";

if ( exists $data->{foo} ) {
    print $data->{foo}{bar};    # 'Can't use string ("") as a HASH ref
+...'
}
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or with undef as value (in which case the missing hash would be autovivfied)

$data->{foo} = undef;

if ( exists $data->{foo} ) {
    print $data->{foo}{bar};    # 'Use of uninitialized value in print
+...'
}
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