Which is why globing can be so lovely, as it includes the file path for you :) I'm not sure if this is any faster than Abigail-II's solution. At least this does only check for the existence of each file once rather than two checks.

my $dir = '/path/to/dir';

print join ", ", grep {
  /(.*)\.mrg\z/ and not -f "$1.did"
} <$dir/*\.mrg>'
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