# qr & no eval
$s = qr/\\/;
$t = "\\\\";
$t =~ s/$s/!/;
print("$t\n");

# qr & eval
$s = qr/\\/;
$t = "\\\\";
eval "\$t =~ s/$s/!/";
print("$t\n");

# qr & no stringification
$s = qr/\\/;
$t = "\\\\";
eval "\$t =~ s/\$s/!/";
print("$t\n");

print("\n");

# qq & no eval
$s = "\\\\";
$t = "\\\\";
$t =~ s/$s/!/;
print("$t\n");

# qq & eval
$s = "\\\\";
$t = "\\\\";
eval "\$t =~ s/$s/!/";  
print("$t\n");

# qq & no stringification
$s = "\\\\";
$t = "\\\\";
eval "\$t =~ s/\$s/!/";
print("$t\n");