Another debugging approach is to ask what all those strings look like when they're concatenated together inside the function.

>perl -wMstrict -le
"my $regexStr = qr{[0-9]+};
 my $suff     = '.dn';
 my $getDir   = 'c:\sh\\';
 ;;
 my $concat = qq{$getDir$regexStr$suff};
 print qq{for debug: '$concat'};
"
for debug: 'c:\sh\(?^:[0-9]+).dn'
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The first thing that catches the eye is that  (?^:[0-9]+) stuff in the middle of everything. What's that about? Clearly, it derives from the regex, but how does it relate to the specification of a path and file name? (Of course, one quickly realizes it is the stringization of the  Regexp regex object – not a string at all – that  qr// produces.)

Seeing the inappropriate nature of the data being fed to the Windoze  dir command is the first step toward thinking more clearly about what one ought to be doing.