my %HoH = (
  yadda1 => {
     key1 => "value1",
     key2 => &GetValue, # returns date string
  },
  yadda2 => {
     key1 => "value1",
     key2 => &GetValue, # returns date string
  },
);
for(  keys %HoH  ) {
    $HoH{$_}{key3}= &Compare( $var1, $HoH{$_}{key2} );
}
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is one way to do it.

        - tye (but my friends call me "Tye")