No, $string and $substring don't contains any "\" to begin with.

# 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");
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my string with  and / slashes.
with  and / slashes

my string with \ and / slashes.
with \ and / slashes

my string "with \ and / slashes".
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