This is done with the signal handler for the alarm call:

use warnings;
use strict;

my $fuse = 10; # Seconds

foreach (<*.mp3>)
{
        print "$_\n";   # Update on progress

        # If the fork() produces a process...
        if (my $pid = fork())
        {
                # ...wait for it to finish.
                wait(); 
        }
        else
        {
                # This causes the process to exit on
                # an alarm call.
                $SIG{ALRM} = sub { exit(); };

                # This sets the timer on the alarm
                alarm($fuse);

                # This runs a command that takes a long
                # time and has to be stopped early.
                system("/bin/sleep 10000");

                # Important to exit() here or you'll
                # keep going through the loop creating
                # more processes!
                exit();
        }
}
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