With warnings enabled, Perl will complain if you try to interpolate an uninitialized or undefined variable, and if there has never been a regex capture to capture group 9, that's what the special variable  $9 will be. See Variables related to regular expressions in perlvar for discussion of  $1 $2 $3 $n   etc.

So one wrong you have done is to ignore the warnings you asked Perl to give you.

>perl -wMstrict -le
"my $sw = '/b';
 my $d  = `dir $sw $9`;
 print qq{'$d'};
"
Use of uninitialized value $9 in concatenation (.) or string at -e lin
+e 1.
'arb_nested_HoH_1.pl
arb_struct_access_1.pl
arb_struct_access_2.pl
...
most_of_8_1.pl
recurring_cycle.pl
recurring_cycle.pl.bak
'
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