# The string these literals produce don't contains any "\".
my $string = "my string with \ and / slashes."; 
my $substring = "with \ and / slashes"; 
print("$string\n");
print("$substring\n");
print("\n");

# This is what the literals should be.
$string = "my string with \\ and / slashes."; 
$substring = "with \\ and / slashes"; 
print("$string\n");
print("$substring\n");
print("\n");

# It works.
$string =~ s/\Q$substring\E/\"$substring\"/gi;
print("$string\n");