Wrap the sub that sends the alerts in a sub that records and checks for the last time that particular alert was sent, and have it do nothing if it was already sent within some specified period:

use constant INTERVAL => 3600;

my %alerts;
sub wrapSendSMS{
    my( $alert, $no ) = @_;

    ## Do nothing if we've sent one recently
    return if exists $alerts{ $alert } 
          and time() - $alerts{ $alert } < INTERVAL;
    
    ## Send it and record when
    sendSMS( $alert, $no );
    $alerts{ $alert } = time;
    return 1;
}
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