$cow = 'moo!';
$_ = 'cow';
print("$cow");          # Prints moo!
print("\$cow");         # Prints $cow
print("\\\$cow");       # Prints \$cow
print("\$$_");          # Prints $cow
print(eval "$cow");     # Error executing "moo!".
print(eval "\$cow");    # Prints moo!
print(eval "\$$_");     # Prints moo!