If disk space is so low that you can't fit a second copy of the file, you need more disk space. Barring that, you could try creating a temporary RAM disk, copying the file into it, reading the file line by line, then writing back an interpreted copy to the original location. Reading from a RAM disk is significantly faster than reading from the hard disk, and the file will only require its actual size in memory instead of its Perl size.

(be sure to create a backup, compressed copy of the file first, however - there's always a tiny chance your comp will crash or something will go wrong, and the copy of the file in the RAM disk will go poof)