To the specific root-cause solutions offered by others, I would add that enabling warnings along with its good friend strict will let Perl at least give you some hints about problems like this:

c:\@Work\Perl\monks>perl -le
"use warnings;
 use strict;
 ;;
 my $directory_name = 'foobar';
 my $dir = \"C:\Users\darkblack\Desktop\A\.$directory_name\";
 print qq{'$dir'};
"
Unrecognized escape \d passed through at -e line 1.
Unrecognized escape \D passed through at -e line 1.
Unrecognized escape \A passed through at -e line 1.
'C:SERSDARKBLACKDESKTOPA.FOOBAR'
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Also, see Quote and Quote-like Operators in perlop for double-quotish escapology info.