Actually, wantarray has three distinct return values: true, false but defined, and undef. These correspond to list context, scalar context, and void context.

sub fred { 
    ...
    unless( defined wantarray ) { ...void context }
    elsif( wantarray )          { ...list context }
    else                        { ...scalar context }
    }
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You then ensure you're calling fred in the right context.

fred();               # void
my $fred    = fred(); # scalar
my( $fred ) = fred(); # list (because of the parens)
my @fred    = fred;   # list
print fred();         # list, print is a list operator
print scalar fred();  # scalar
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For instance, you might look at Hook::LexWrap (or my TPJ article on it: "Wrapping Subroutines to Trace Code Execution".