Variables & sigils

Dynamic variables $*

Full

Variables looked up through the call stack, not the lexical scope.

A variable with the * twigil ($*name) is dynamic: a called routine sees the caller's value, not one from where the routine was written. It's controlled action at a distance — for context like the current output handle or a request-scoped setting.

Visible down the call stack #

my $*greeting = "hi";
sub greet { say $*greeting }
greet();

Output

hi

greet never declares $*greeting, yet finds it — because the lookup walks the dynamic (caller) chain, not the lexical scope.

Rebinding for an inner scope #

Because the lookup follows the caller, redefining a dynamic variable in an inner block changes what calls made from that block see — and only those. When the block ends, the outer binding is back.

my $*g = "outer";
sub greet { say $*g }
{
    my $*g = "inner";
    greet();
}
greet();

Output

inner
outer

The first greet() runs while the inner $*g is in effect, so it prints inner; the second runs after the block, seeing outer again. (For an automatic restore without a new lexical scope, use temp.)

Notes #

Contrast with a my variable, which a sub only sees if it's in the sub's lexical scope; a dynamic variable is found via whoever called, however deep.
The built-ins use this: $*OUT (standard output), $*IN, $*ERR, $*PID, and $*CWD are dynamic, so you can temporarily rebind $*OUT to capture output.
Rebinding in an inner scope ({ my $*greeting = "yo"; greet() }) affects only calls made from within that scope — the change is dynamically scoped.