The best way (to my knowledge) to do that is to use backticks, assigning the result to a scalar (the output doesn't have to be a single line, or a single word or anything. For example:

$result=`/bin/tail -15 /var/adm/messages`;
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You should be very careful though, if you decide to pass to the command options from the user's input (for example through a CGI script). But that's a fact, either you use backticks or system.