This approach introduces a cyclic reference with $fac.

use Devel::Cycle;

my $fac;
$fac = sub { 
    my ( $n ) = @_;
    return 1 if $n == 1;
    return $n * $fac->( $n - 1 );
};

find_cycle($fac);

__END__

Cycle (1):
                      $A variable $fac => \&A
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