You can pass a reference to a sub, and let other code call that whenever you like...

use strict;
use warnings;
{
  my $subref = sub { print "Hey, Hey!\n"; };
  Foo($subref);
}
exit 0;

sub Foo
{
    my $callback = shift;
    print "Foo started\n";
    $callback->();
    print "Foo is done\n";
}
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