readdir returns file names, not paths, so -z could be failing to find the (right) file to check.

By the way, it would be faster to check -z second (since it requires an io call), and it would be better to avoid needless captures.

opendir(my $dh, $dir)
    or die("opendir: $!\n");

my @files =
    grep { ( !/\.(?:txt|prn|csv)\z/ && /$SrcPattern/ ) || -z }
    map "$dir/$_",
    readdir($dh);
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