$srcroot = "/some/(dir)";
$destroot = "/some/other/dir";
$srcdir = "/some/(dir)/with/more/sub/dirs";

($extra_path = $srcdir) =~ s/^$srcroot//;
$destdir = $destroot . $extra_path;

print "extra_path: $extra_path\n";
print "destdir: $destdir\n";