tools: Remove warning from newer versions of Perl.

Replace \1 with $1 in substitution strings.

src/tools/fullPathName.pl

@@ -29,7 +29,7 @@ $Getopt::Std::OUTPUT_HELP_VERSION = 1;
 my $path = AbsPath(shift);
 
 # Escape shell special characters unless on Windows, which doesn't allow them.
-$path =~ s/([!"\$&'\(\)*,:;<=>?\[\\\]^`{|}])/\\\1/g unless $^O eq 'MSWin32';
+$path =~ s/([!"\$&'\(\)*,:;<=>?\[\\\]^`{|}])/\\$1/g unless $^O eq 'MSWin32';
 
 print "$path\n";