I think that MacPerl::GetFileInfo should work.

MacPerl::GetFileInfo(FILE) 
In scalar context, returns the file type. In array context, returns (creator,type). Examples: 
    MacPerl::GetFileInfo(yin);


Returns "TEXT" or ("MPS ", "TEXT"). 

- From Macperl. 

You may want to look at this MacPerl Primer as well.

good luck.