You could do that, sure... in that case you just need to use stat to get the mode of the file.

my $mode = (stat "foo")[2];
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But I think you could use umask, still... and it may make it easier. Try something like this:

my $mode = (stat "foo")[2];
my $mask = 777 -
    join '', (($mode&0700)>>6, ($mode&0070)>>3, ($mode&0007));
umask oct $mask;
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I'm not sure if this is absolutely right (please someone correct me if it isn't), but it seemed to work for the test cases I tried.