You can do it if the variable to tie is passed as an argument, since the elements of @_ are aliases (not copies) of the arguments.

sub ModName::TIESCALAR {
   my $class = shift;
   my $self;
   return bless(\$self, $class);
}

sub test {
   tie $_[0], 'ModName';
}

my $var;
print(tied($var) ? 'tied' : 'not tied', "\n");  # not tied
test($var);
print(tied($var) ? 'tied' : 'not tied', "\n");  # tied
[download]

Otherwise, you have to return it as a reference.

sub ModName::TIESCALAR {
   my $class = shift;
   my $self;
   return bless(\$self, $class);
}

sub test {
   my $var;
   tie $var, 'ModName';
   return \$var;
}

my $ref = test();
print(tied($$ref) ? 'tied' : 'not tied', "\n");  # tied
[download]

You can't make a copy of a tied variable. You'll only be making a copy of the value its FETCH returns.

sub ModName::TIESCALAR {
   my $class = shift;
   my $self;
   return bless(\$self, $class);
}

sub ModName::FETCH { }

tie my $src, 'ModName';
print(tied($src) ? 'tied' : 'not tied', "\n");  # tied

my $dst = $src;
print(tied($dst) ? 'tied' : 'not tied', "\n");  # not tied
[download]

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