# code not tested
sub test_this {
  my $stuff = call_to_external_service();
  return $stuff;
}

#...

# subroutine mock for a unit test, e.g.,
{
  no warnings qw/redefine/;
  my $expected = { some => 'expected result'};
  local *call_to_external_service = sub { return $expected, };
  # now call a test or something that calls
  is_deeply test_this(), $expected, qw{whatever...};
}