Translation of those lines:

  # %main:: is the main symbol table (the :: are part of
  #         the variable name in this case)
  # lookup the entry for "_<$fname" (note that symbol
  #    tables can contain not-normally-valid identifiers
  #    that are accessible only via the symbol table hash or
  #    via symbolic references)
  # in this scope only, alias *dbline to *{"_<$fname"}.
  #    this will alias $dbline, @dbline, %dbline, etc.
  #    though I'm guessing that only the filehandle is
  #    actually wanted.

  local(*dbline) = $main::{'_<' . $fname};

  # for this scope, hide $F, @F, *F, etc. and use fresh ones
  # typically used to say things like open F, ... without
  # worrying that some other code is using the F filehandle.

  local(*F);

  #  Make the GUI:: symbol table (aka package) an alias to
  #  Win32::GUI::
  *GUI:: = \%Win32::GUI::;
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