Something like the following might work (untested, caveat emptor):

#!/usr/bin/perl -w
use strict;
use POSIX qw(:sys_wait_h);
my $TIMEOUT=180;# or whatever
my $start=time;
if (my $pid = fork) {
    #parent
    while (1) {
        if (waitpid($pid,WNOHANG) == -1) {
            # child is done
            exit;
        } 
        if ( time > ($start + $TIMEOUT)) {
            # time has expired, kill the child
            # this is probably too simplistic, 
            # as the child could ignore the SIGTERM
            kill(15,$pid);
            exit;
        }
        sleep(1);# avoid the tight loop
    }
} else {
    # child
    exec('/some/program');
}
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