my $tied_variable;
tie $tied_variable,
    'Tie::Scalar::Function' => sub { ok( defined $_[0], "got something -> $_[0]"
 ) };

warn 'here1:',$tied_variable;

open(my $fh, '>', \$tied_variable);
print {$fh} 'this is a test';

warn 'here2:',$tied_variable;