"I mean C:\\temp is an invalid path under Windows."

While I'm not disagreeing with that absolute statement, C:\\temp may be required under a Windows environment. I came across an example of this fairly recently.

Consider a pp command on cmd.exe something like this (using Strawberry Perl):

C:\some\dir>pp @long_list_of_options.txt -o filename.exe filename.pl
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Options (in long_list_of_options.txt) which include paths cannot be written like this for example:

--link=C:\path\to\some.dll
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The parts like \X will be interpreted as backslashed escapes: resulting in just X, or special characters (e.g. newline, tab, and so on). The example option I showed would end up like:

--link=C:path    osome.dll
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In order to get it to resolve to the example shown, you'd need to write:

--link=C:\\path\\to\\some.dll
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That doesn't mean that an XY Problem doesn't exist; it just shows that it might not exist. Having said that, the terminal \\\\ certainly looks highly dodgy.

I think you need to make your solution either smarter or not as helpfull, because Windows file paths are allowed to start with \\ in UNC notation. For example, this is a valid UNC filepath:

\\servername\c$\foo.txt
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use strict;
use warnings;

my $dir;
while ( (print "enter directory; "), $dir=<STDIN> and $dir =~ /\\\\|\/
+\//g)
{
   print "invalid directory syntax... double ", '// or \\\\', " not al
+lowed!\n";
}

$dir =~ s/^\s*|\s*$//g;  #remove leading and trailing spaces
print "INPUT LOOP says $dir\n";

__END__


C:\Monks>perl CommandLoopNoDoubleSlash.pl
enter directory; C:\\x
invalid directory syntax... double // or \\ not allowed!
enter directory; D:\x\temp//
invalid directory syntax... double // or \\ not allowed!
enter directory; C://x
invalid directory syntax... double // or \\ not allowed!
enter directory;     C:/X
INPUT LOOP says C:/X
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