Of course cron might be the best way, but if you have a kind of server-program, which is always running, you could use alarm to send a alarm signal to your program.
Then you can call a subfunction when the alarm signal is received.
i.e.

alarm 900;  #send alarm after 900 sec
$SIG{ALRM} = \&alarm_fnct;

# Here should be the code of your allways running server

sub alarm_fnct {
 print "Get alarm Signal";
 ...
 alarm 900; ## and again call alarm after 900 sec.
}
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Remember this is only usefull, if you have a kind of server program, and you want to call every 900 sec. a special sub, i.e. to read the contents of a directory or something like that.

Hope this is not 'off topic' :)

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