See fork and exec.

my $cmd = 'mplayer command line';
my $child_pid = fork();
die "Can't fork: $!" if ! defined $child_pid;
if ( ! $child_pid ) {
    exec $cmd or die "Can't exec '$cmd': $!";
}
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At that point, you have to remember to wait for the child at some point (with a $SIG{CHLD} handler). The documentation for fork discusses this.