You might want to pass in a code ref instead of a hash ref, and then in your function you can say:

my $signature = $custom_coderef ? $custom_coderef->() : default_sig();
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The code ref itself can require any modules it needs, eg:

my $coderef = sub{
  require Foo::Bar;
  my $signature = "blah";
  return $signature;
}
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