use POSIX ":sys_wait_h";

# e.g.:
my ($pid, $listing) = pidqx( "ls" );

sub pidqx { # returns pid + the usual backtick result
    my $pid = open my $ph, "$_[0] |" or die "$!: $_[0]";
    local $/ = undef();
    my $backtick = <$ph>;
    close $ph;
    waitpid $pid, 0; 
    return ( $pid, $backtick );
}
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my $pid = open my $ph, "| $_[0]" or die "$!: $_[0]";

Your open is opening for write instead of read. The command's output goes to STDOUT instead of to your variable. Try this:

sub pidqx { # returns pid + the usual backtick result
    my ( $cmd ) = @_;
    my $pid = open my $ph, '-|', $cmd or die "$!: $cmd";
    local $/ = undef();
    my $backtick = <$ph>;
    close $ph;
    return ( $pid, $backtick );
}
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Also, you don't have to wait for the command to finish because close does that for you.