What I've done that covers many of my use cases, is:

1. Watch & wait for the first type of file to appear.
2. Sleep for a few seconds to a few minutes depending on the task
3. Proceed to verify the other matching files exist, and carry on with the task, else loop back to 1

Sometimes the verification is a check to see if the file has grown larger; if so, then it goes back to sleep repeatedly until the download/copy is finished.

For checking the names, you may want to use a regex to capture the relevant part of the filename, and then look for specifically named files using that.

For example,

if ($file =~ /alphat_(.*)\.zip)
{
  my $key = $1;  # whatever was between alphat_ and the .zip bits.
  if (-f "numerict_${key}.zip && -f "updatet_${key}.zip")
  {
     print "Caught 'em all!\n";
  }  
}
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