afresh1 .. You probably meant that to be

       map { $self->{$_} = $initdata{$_} } keys %initdata;
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since there is no indication of the existance of a %self hash, or the relevance of such a hash to the object.
Also, there is not need for the "if" since empty "keys" will effectively convert the map to a no-op.
So your code is equivalent to the hash slice jdporter wrote.

My preference would be to simplify it to :

  %$self =  (%$self, %initdata);
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which, although representing infinitasmally more overhead, has an elegant simplicity to it.

If you really wanted to be non-destructive of $self or $_[0], you could:

   $self->{$_}=$initdata{$_} for keys %initdata;
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which, to me, is slightly more readable/DWIM, compared to a "map" whose result is thrown away.