Braces always create a new scope, and my always creates a lexical variable, which is undef by default -- or at least, should be. In practice the behavior of my $x = 'foo' if $bar is undefined if the condition is false, and is documented as such.

Since the scope of a lexical variable begins at the statement after the statement that defines it, the ternary operator does what you want: my $self = $ARGV[0] eq 'Fido' ? OO->new( $dog ) : $self;. The $self in the assignment statement refers to the variable outside the braces, since the variable inside the braces is not yet in-scope.