alarm would be one way:

    my $OLD_SIG = $SIG{ALRM} if $SIG{ALRM};
    $SIG{ALRM} = sub { die "timeout" };

    eval {
        alarm( 30 );
        # do what needs to be done
        alarm( 0 );
    };

    if( $@ ) {
        if( $@ =~ /^timeout$/ ) {
            print STDERR "A timeout occurred";
            alarm(0);
        } else {
            print STDERR "Some other error occurred";
        }
    }

    $SIG{ALRM} = $OLD_SIG if $OLD_SIG;
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-derby

update: Whoa! Mis-read your question as not more than 30 seconds. I'm not sure why you would want it to run for at least 30 seconds but you could sleep.