$foo = "\tfoo";
$_ = ">>ha<<\n";  s/ha/$foo/;      print;
$_ = ">>ha<<\n";  s/ha/\Q$foo\E/;  print;
quotemeta $foo;  # update: this is doing nothing!
$_ = ">>ha<<\n";  s/ha/$foo/;      print;

$foo = '\tfoo';
$_ = ">>ha<<\n";  s/ha/$foo/;      print;
$_ = ">>ha<<\n";  s/ha/\Q$foo\E/;  print;
quotemeta $foo;  # update: this is doing nothing!
$_ = ">>ha<<\n";  s/ha/$foo/;      print;