And just to be cute, this uses 2 pipes and is shorter (but I would not write it this way).

    #!/usr/bin/perl -w
    use strict;
    use English;
    use autodie;
    
    sub f { open(my $h,"/bin/ls $_[0]|") ; print "--\n",(<$h>),"--\n";}
    
    open(my $o,">/tmp/out.txt") ; select $o ; f("/tmp") ;
    open($o,"| wc -l ")         ; select $o ; f("/tmp") ;