At the time I write this, the question and all answers have a Useless Use of Cat. And that's actually complicating the solutions. If you get rid of the unneeded pipe, you can switch to a multi-arg system call, and then you can pass a string without worrying about what it contains:

system "grep", "-e", $_, "/etc/mail/aliases";
[download]

And in this case, it saves not one process, but two!

Beware the useless use of cat.

-- Randal L. Schwartz, Perl hacker
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update: updated URL per notice.