You've got to either pass a reference to a variable explicitly, or define the function as something that takes a reference. Here's two examples, with usage:

# This function takes a value by reference,
# which is the responsiblity of the caller.
sub fref
{
        my ($var) = @_;

        $$var = $$var . " Python";

        return $$var;
}

# This function defines a prototype which takes
# a reference instead of a scalar. This way the
# caller doesn't have to do anything special.
sub fprot(\$)
{
        my ($var) = @_;

        $$var = $$var . " Python";

        return $$var;
}

# The reference version requires a backslash before any
# passed parameters.
my $test = "Monty";
print fref(\$test),"\n";
print $test,"\n";

# While the prototype means that you don't have to worry
# about doing anything special.
my $test = "Learning";
print fprot($test),"\n";
print $test,"\n";
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