Unix systems already have a standard command line utility to do this. It's called "paste":

paste file1 file2 file3 > merged_file
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And like all good unix tools, of course, it has been ported to windows (by cygwin, ATT Research Labs, GNU, and others).

But using perl for this (as demonstrated by BrowserUK) is still fun and rewarding in its own right -- e.g. in case you need to do character encoding conversions while merging files, or manage column widths or modify column contents in intelligent ways. (You could use "paste" in combination with other standard unix tools to do these things, but at that point, it's not so different from just writing a little perl script.)