Another approach in newer perls gives something that looks very C-like in use. Make foo an lvalue accessor which takes no arguments:

BEGIN {
    my $foo = "default";
    sub foo () : lvalue { $foo }
}
print foo, $/;
foo = "special";
print foo,$/;
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The BEGIN block is only needed to get the "default" value in early (completely unneeded in this simple case). In a module, you can leave 'foo' off the @EXPORT_OK list so it's perlishly private to the module package.

For thread safety, you'd want the 'locked' attribute, besides 'lvalue'.

Update: Added clarification on the BEGIN block, and an unwise adverb.

After Compline,
Zaxo