There's an alternative to do this kind of thing with the standard unix text utilities instead of perl. Of course, you might not want to do that if you want to learn perl.

So here's the method. 

[am]king ~/a/tm$ cat emails
user01 user01@emailaddress.com
user03 user03@emailaddress.com
user04 user04@emailaddress.com
[am]king ~/a/tm$ cat names
user01
user02
user03

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First, you sort the two files on their first field. They appear sorted in this example, but it's always best to go for sure. 
[am]king ~/a/tm$ sort -k1 emails > emails.sort
[am]king ~/a/tm$ sort -k1 names > names.sort

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Then, you use join to find the unpairable names in emails.sort. 
[am]king ~/a/tm$ join -v1 -o0 emails.sort names.sort
user04

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You might also be able to use the comm utility instead of join. Update 2009 sep 2.

See Re^2: Joining two files on common field for a list of other nodes where unix textutils is suggested to merge files.

In reply to Re: Seeking guidance on how to approach a task by ambrus
in thread Seeking guidance on how to approach a task by mark_nelson

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